While I support the answer of Mwr247 for important context (even if it largely lifts from another source that lacks a clear CC license), I came across an interesting answer for this question years ago at a diverse table of people from many countries.
The Turkish representative had brought a box of Turkish delight, imported from Turkey, and thus labeled in its original Turkish language ("Lokum"). This was touted as the real thing; I'd finally get to learn after years of waiting just what Edmund (Lewis) was referring to.
The Danish representative, arriving a few moments later, laughed and commented that in his language, the word means "toilet."
So even linguistically, we were set up for the experience of the dessert being a welcome treat but not nearly as amazing as Edmund described it, in comparison and context with a modern peacetime of relative abundance where a wide variety of sweets and rich foods are relatively easy to come by.
If Lewis knew this, this particular choice of treat could have been a hidden reference to something which seems quite attractive on its surface but has a much dirtier true meaning underneath. With evacuated Danish children in wartime England, real Turkish delight something that would have to be imported (according to Leah Schnelbach's article quoted by Mwr247), and the higher average number of languages Europeans are familiar with (compared to residents of other English-dominant countries), it would not necessarily be that unusual for Lewis or at least some of his readers to pick up on this connection.
His writing style puts meaning on so many different levels and layers, that I would not be entirely surprised if this was intentional. Also, he could not have conveyed that meaning with any other choice of treat, so it lends a bit more weight to the possibility that the extra layer of meaning may have been intended.
In a later reading of the particular section quoted in the question, I also interpreted "Turkish Delight" as a code for addictive drugs (e.g. opiods) that also make one forget one's manners, think only of trying to shovel down as much as one can, where the more one consumes the more one wants to consume. Lewis's goal of embedding morals and lessons important for young people to learn would likely have included strong cautions against this sort of substance, even if is something that sounds exotic and tempting, because such drugs lead to one's downfall and hurt one's family.