I'm not that extremely bothered by the stories Blizzard has told over the years, but I will say
that is definitely bullshit. All Tyrande did was kill one of Sylv's Valkyrie's, which although very
important, isn't nearly enough to avenge the destruction of Teledrasil.
And besides, in that fight, she also lost 2 of her Night Elven warriors, now raised into undeath
to serve Sylvanas. If anything, she lost that fight far more than she won.