I'm just going to hedge this out. But has anyone read about the Silver Hand backstory that involves Tyr and Odyn?
He also says that Helya imprisoned on the Broken Isles and will return to basically Northrend to go and see Tyr back in Ulduar. He states it all at the end when Helya is killed in Trial of Valor. If you don't believe me then google him in wowwiki.
So I personally think he knows but views Sylvanas as insignificant to the grander schemes of things because he's too busy trying to escape the Isles. When Greymane supposedly stopped Sylvanas's plans he probably was like well, it's not that important.
His significance as a character will probably shine when we venture back to Northrend in a coming expansion or whenever. When he goes to Northrend and sees Yogg has been a naughty boy and his fellow titanic keepers weren't doing their jobs and most of all Tyr's absence.