Not exactly an argument against, but when Exile asked the Jedi Masters: "Am I responsible for Katarr?", the Jedi Masters say, "No, of course not. But something different..." Again, not an argument against, but still.
Here is my beliefs concering this:
1) This could remove any sort of mystery behind Nihilus. Some things are better left unexplained, altough this story could finally boost Nihilus' rating.
2) "Huh?"---Brand New K3 Player. You know they exist, people play K2 before they play K1 you know. Same thing here. They might get confused with all the reference to Nihilus here. Too much focus on the events in TSL would sort of make K3 pretty hard to stand alone by itself.
3) Tying into that, if K3 were to focus on the True Sith, then too much discussion about Nihilus would remove that focus on the True Sith, making the story a bit weaker. Unless, this fact would play a key role, as you had it did in your K3 storyline, which I do somewhat oppose...
4) If this turns out to be the truth, you did realize that you just spoiled the entire 'plot twist' for me? It's like telling ESB fans before they watch ROTJ, "Did you know Darth Vader would end up redeeming himself to the Light Side because he loved his son and would rather fall to the LS than have his son get hurt?" What sort of punishment you should get for that?
My view is that the developers won't say anything more about Nihilus. He's gone, he's dead, forever. He existed to show the Player (not the Exile) the power of Force Bonds and how, if used for the Dark Side, can threaten the whole galaxy. There is no need. I like to know a little bit more about what Kreia meant when she said "True Sith" please.