What led Qui-Gon to conclude that Darth Maul was a Sith?

Question

This question is similar to this one pertaining to Mace Windu's assessment, but there are different circumstances that I think are noteworthy, and therefore justify a separate question. When Mace Windu explained his conclusion to Yoda, they had now seen what Darth Maul was capable of, as he had fought two Jedi, and successfully killed one of them (a very well-experienced Jedi Master).

However, when Qui-Gon first fought against Maul, it was the first time that any Jedi had seen him (at least as far as what the films show; and none of those present during Qui-Gon's subsequent meeting with the Jedi Council had any prior knowledge of Maul), and Qui-Gon spent very little time fighting against him at that point. When Qui-Gon escaped by boarding the Queen's ship, he had this exchange with Obi-Wan:

OBI-WAN : What was it?

QUI-GON : I'm not sure... but he was well trained in the Jedi arts. My guess is he was after the Queen...

And later, when he relayed his experience and his conclusion to the Jedi Council:

QUI-GON : ...my only conclusion can be that it was a Sith Lord.

KI-ADI : Impossible! The Sith have been extinct for a millennium.

MACE WINDU : I do not believe the Sith could have returned without us knowing.

So, considering the Council's skepticism of Maul's affiliation due to how long the Sith have been absent from the Jedi's affairs in the galaxy (and thought to have been long-extinct), what was it that led Qui-Gon to believe that Maul was a Sith Lord? I understand that it was probably easy to conclude that he wasn't a fallen Jedi, but why not some other type of Dark Jedi? The definition of Dark Jedi (at least according to Wookieepedia) is a Force-sensitive that adheres to the Dark Side, but isn't a Sith. In fact, if we rely on Legends, there were a few notable examples of Dark Jedi, such as Allya, then there was Kibh Jeen during what became known as the Dark Jedi Conflict, and, most notably, there was Qui-Gon's former Padawan Xanatos. And, on another note, Maul was a Dathomirian Zabrak, not a Sith Pureblood.

Based on what Qui-Gon knew at the time, wouldn't it have been the more logical to conclude that Darth Maul was actually a previously unknown Dark Jedi?

Note: While I drew upon Legends information in forming my question, an answer from either Canon or Legends is acceptable (depending on what information is available). If possible, answers for each are appreciated.

Answer 1

In-Universe

The film's junior novelisation is the only source that really seems to address this, and even then only obliquely. Qui-Gon apparently described his fight with Maul at considerable length., seeming to consider this to be highly relevant to his goal of convincing the Jedi Council that he had been attacked by a Sith.

He touched only briefly on the events on Naboo and the Podrace, but he described the fight on the Tatooine dunes in great detail. Then he finished, “My only conclusion can be that it was a Sith Lord.”

_{^{Phantom Menance: Junior Novelisation}}

That being the case, it would presumably come down to three main factors;

• Qui-Gon has studied the Sith and would have some knowledge and understanding of their teaching and training techniques.

• Maul's fighting style (and sheer competence) is almost at Jedi levels, indicating extensive schooling.

• Maul was emanating a deep hatred for the Jedi and passed up chances to attack the ship in favour of attacking his Jedi opponent.

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Out of universe

Wookieepedia goes into this further, but the very short answer is that the concept of "Dark Jedi" (e.g. Force-sensitives who've managed to self-train and then turned to the dark side of the Force) isn't something that existed in the main Star Wars canon at the time the film was made.

... a great deal of controversy has arisen over the issue of who is and is not a Dark Jedi. Perhaps due to uneven usage within canonical sources, many fans have argued that Dark Jedi describes only those dark side users who had previously been a Jedi or were at some point in the process of being trained as a member of the Order.

"We avoid [Dark Jedi] when we can. Dark Jedi definition still refers to any dark side user." ―Leland Chee

"A dark Jedi is anyone who practices the dark side of the Force whether or not they were a former Jedi." ―Leland Chee