What led <PERSON> to conclude that <PERSON> was a Sith?
This question is similar to this one pertaining to <PERSON> assessment, but there are different circumstances that I think are noteworthy, and therefore justify a separate question. When <PERSON> explained his conclusion to <PERSON>, they had now seen what <PERSON> was capable of, as he had fought two Jedi, and successfully killed one of them (a very well-experienced Jedi Master).
However, when <PERSON> first fought against <PERSON>, 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 <PERSON> subsequent meeting with the Jedi Council had any prior knowledge of <PERSON>), and <PERSON> spent very little time fighting against him at that point. When <PERSON> escaped by boarding the <PERSON>'s ship, he had this exchange with <PERSON>:
<PERSON> : What was it?
<PERSON>-<PERSON> : I'm not sure... but he was well trained in the Jedi arts. 605
My guess is he was after the Queen...
And later, when he relayed his experience and his conclusion to the Jedi Council:
<PERSON><PERSON><PERSON> : ...my only conclusion can be that it was a Sith Lord.
<PERSON>-<PERSON> : Impossible! The Sith have been extinct for a millennium.
<PERSON> : I do not believe the Sith could have returned without us knowing.
So, considering the Council's skepticism of <PERSON>'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 <PERSON> to believe that <PERSON> 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 <PERSON>, then there was <PERSON> during what became known as the Dark Jedi Conflict, and, most notably, there was <PERSON> former Padawan <PERSON>. And, on another note, <PERSON> was a Dathomirian Zabrak, not a Sith Pureblood.
Based on what <PERSON> knew at the time, wouldn't it have been the more logical to conclude that Darth <PERSON> 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. 605