That is so brain bendingly wrong I don't even know how you could say it with a straight face. If the RTS games were non-canon how could WoW exist? The only parts of the RTS series that are non-canon are the parts contradicted by later canon with his only by virtue of it being more recent not as a result of WoW having some primacy (which it certainly doesn't since the supposed canon supplementary books and novella regularly build upon and contradict the game).
Further Arthas' soul being the first one stolen by frostmourne is a foundational aspect of the character, I can't think of anything in wow that could even vaguely be interpreted as contradicting this.