While I in essence agree with what Borbarad is saying regarding the outdated nature of the source in question and its lack of compatability with material that is more recent, prevalent and of higher standing in canon, as well as how the source goes out of its way to present the aforementioned quote in a questionable manner using the "It is believed" qualifier, I think he may be missing the point with regards to some of the other points of discussion.
Of course a statement given in 1993 can only refer to information present at that very time, unless somebody wants to argue, that the person writing it down, was gifted with some nice forsight ability.
Or, and as it would seem is more likely upon further examination of the quote, the statement was made not with prior knowledge of then as of yet unknown material, but the prior assumption of what the then as of yet unknown material could potentially entail with possibly a later retroactive establishment of the character in light of more up to date information in mind.
"It is believed that he has mastered nearly all the known powers, previously unknown powers, and devises new ones at his pleasure."
If you look at the quote the intended perspective would almost surely be an in-universe one given that the qualifier "It is believed" wouldn't reasonably match the credibility of an LFL authorised writer, not only expected to possess a comprehensive understanding of canon but also granted the authority to establish canon, nor would it match the style of an out-of-universe narrative, speaking introspectively in the third person, not to mention the distinction between known and previously unknown powers which would more accurately reflect a fallible in-universe awareness; an in-universe perspective assumes a different level of knowledge to what would have been presented to an out-of-universe audience, and the implications behind the scale of the powers mentioned suggest a quantity beyond what had been established in canon up until that point in time. Moreover with regards to Palpatine devising new techniques at his pleasure, correct me if I'm wrong but at that particular point in time I don't believe we had been particularly well informed regarding anything specific that Palpatine had created, not to mention again the implications behiond the scale of his inventions; clearly the most realistic interpretation of the statement is that it assumes a level of information that hadn't outwardly been established as canon up until that point in time.
....he will find out that the quote is part of the game mechanics contained within the sourcebook. To be precise: The sole fuction of that quote is to given an explanation for Emperor Palpatine's "Alter Force" stat - which is explained here. As, in general, game mechanics are considered to be not canon, this quote can be ruled out right here already.
Given that the quote in question details information immaterial to the RPG system at play (the length of time of Palaptine's studies, his invention of new techniques), and where it does detail relevant information it leaves it largely undefined (mentioning that he has mastered nearly all these powers but not defining an exact quantity at which point you could act off of the information, casting doubt on the information by mentioning how it is simply believed to be the case), I find your interpretation of it as a detail regarding gameplay mechanics and not storyline dubious at best.
Regarding RPG sourcebooks, you have yet to definitively establish that they are marketed entirely towards people who plan to use it solely as part of the game, and you have yet to explain how a significant, prevalent interactive element to the sourcebook prevents it from establishing canon fact in areas where no interactive element is present, such as with the aforementioned quote.