Valkorian's still described as a sith in sourcebooks. Besides, even Vitiate powerscaling leads us to the same conclusion.
Also, Sidious is “the greatest master of evil to ever use the Sith power”. So no matter how powerful Valkorion becomes or whichever loopholes Bioware tries to jump him through, he's still an entity that once used Sith power, and thus can never be on Palpatine's throne. 😈
Just accept it.
Who cares, he was once a Sith but now he isn't. This is canon to the game and is repeatedly made clear over and over in dialogue and other sources.
Sidious being "the greatest master of evil" isn't relevant to his force power, that has no direct reference in the quote. Don't know why you posted it, it's useless.
Valkorion is a liar. 3rd person sourcebooks are not.
That quote clearly refers to strength in the Force, unless if you think it was referencing his political control or something, which wouldn't really fit into the context of things.
Also he "had succeeded where all others had failed in taming the Dark Side", so he's still > Valky. Sorry mate.