The main source of that video is the the 2017 book Flora of Middle-earth by Walter S. Judd and Graham A. Judd.
This book spends some time discussing the possibility of althelas being basil, but ultimately rules it out since basil "would never be found in shaded thickets" and "cannot be described as hoary".
Some have compared kingsfoil to sweet basil (Ocimum basilicum, of the plant family Lamiaceae). This mint is highly aromatic (containing thymol- essential oils and also linalool, a terpene alcohol) and is used as a culinary and medicinal herb. The name “basil” is derived from Old French, basile, from Latin basilicum and Greek basilikon, meaning “royal plant,” from basileus (king), perhaps because the plant was used in making royal perfumes. Thus, like
kingsfoil, basil is linked to kingship, and both are highly aromatic herbs. However, sweet basil grows only in sunny, open habitats (so would never be found in shaded thickets, as is the case with kingsfoil); also, it is native to southern Asia and, like nearly all of the ca. 65 species of Ocimum, is a plant of subtropical to tropical regions. The leaves of O. basilicum are intensely green to olive or yellowish green, so cannot be described as hoary.
Walter and Graham also discuss the merits of wintergreens (leaves are the wrong shape and color), and plantain (wrong color and smell). They conclude that althelas is probably comfrey, despite its lack of aroma and it's seasonal color changes.
In summary, we think it most likely that the primary inspiration for Tolkien’s athelias was the medicinal herb comfrey— because it best matches the morphological characters of kingsfoil— but he may have added elements of other plants, such as the persistently green leaves of wintergreen and the sweet fragrance of basil (or some other mint).
It should be noted that while Walter and Graham are skilled botanists, their knowledge of Tolkien is not as good, and their book has been criticised as such. Take their claim that Athelos even has a real world counterpart with a grain of salt.
One may well wonder how productive it is, finally, to declare kingsfoil/athelas ‘most likely a species of Symphytum (and thus a relative of the medicinal herb comfrey)’ (198). The name athelas is also oddly misspelled athelias in multiple locations, perhaps suggesting less attention to detail in their examinations of the entirely fictional plants than in their real-world botanical explications. Strangely, such fascinating plants as the Ents are discussed at length only in a tangent located in, of all places, the section on moss.
Overall, I find that the authors of this book make two fatal mistakes: in assuming, first, that Middle-earth operates according to their understanding of planet Earth and the natural laws that govern it, perfectly mirroring Earth except where Tolkien specifically indicates otherwise, and, second, that the texts of Tolkien's writings grant us unmediated access to that world. As a work of botanical description, Flora of Middle-earth is dizzying in its attention to detail and highly compelling in its capacity to awaken a greater appreciation for the beauty of the natural world and the diversity of plant life referenced by Tolkien. As a work of literary scholarship and interpretation, however, the book is regrettably unsophisticated, occasionally to the point of naiveté.
Journal of Inklings Studies Volume 9, Issue 1
the botanical agenda is higher on the list of priorities than a study of Tolkien’s works as such. This feeling is accentuated when the authors casually state that in order to appreciate these plants “we need a few descriptive terms” (37)—and then proceed to explain no fewer than 155 of these terms over the next ten pages, without any reference to Tolkien’s works. Personal experience tells me that one can be perfectly appreciative of plant-life without knowing the meaning of such terms as epiphytes, rhizomes, or lenticels (or indeed most of the other terms on the list), and though readers are free to skip the chapter if they feel we can do without these concepts, the suspicion this far into the book is that the authors are using Tolkien as an excuse to stimulate an interest in botany, in a vein similar to that employed by Brian Bates in The Real Middle-earth: Exploring the Magic and Mystery of the Middle Ages, J.R.R. Tolkien and “The Lord of the Rings” (2002) which, however fascinating in itself, delivers a lot of Anglo-Saxon history and next to nothing of the Tolkienian content promised in the subtitle.
Journal of Tolkien Research Volume 4, Issue 2
Primary-world distribution enables comparison with Middle-earth environments but, as with economic uses, this treatment often confuses the essential contextualizing boundary between them. In addition, the morphological details that conclude each entry tend to distract attention from its specific Middle-earth significance. ... This method of analysis becomes a particular issue with the speculative treatment of plants that are unique to the legendarium ... Walter Judd frequently notes the healing properties of primary world plants, such as nettles and marigolds, as he deals with them. He treats the fictional healing herb athelas similarly, under its vernacular name kingsfoil. The important combination of athelas with the hands of a king, while mentioned, ignores Tolkien’s juxtaposition, surely significant for Aragorn’s biography, of beliefs in the mythic power of kings to heal physically and metaphorically with
Ioreth the healer’s comment on the inconspicuous appearance of the plant, “if I were a king, I would have plants more bright in my garden” (RK, V, viii, 140). In keeping with the rigidly scientific approach of Flora of Middle-earth there is, instead, lengthy speculation that the plant indicated might be comfrey (symphytum) or plantain (plantago). While these important healing herbs may illuminate the beneficial effect of athelas in terms of primary-world botany, the entry could have usefully noted that plantain was known as “waybread” (Gilliver, Marshall, and Weiner 208), and was one of the nine herbs
recommended in the Anglo-Saxon healing spell against the nine flying venoms (Pettit 41). .... The Judds admit that they quote from only a few critical works on Tolkien, and the bibliography confirms their selective reading.
Tolkien Studies Volume 17