I don't think Sela knew the circumstances of PrimeYar's death, just that it was meaningless? And perhaps not even that? Alt-Guinan wasn't big on the specifics and they didn't have any other access to the Primeline.
You may have a point here. Even if she knew approximately when that death occurred, she couldn't possibly know the exact mission and planet that caused her death. They did a lot of things that could have had some of them killed during that year.