Where they basically said that killing 1 of Sylvanas’ Val’kyr was Tyrandes and the Night Elves’ revenge for Teldrassil. Later we even learned that Sylvanas can recreate those Val’kyr whenever she wants:
it even makes sense looking at it afterwards, because Tyrande has done absolutely nothing after 8.1 and is completely fine with the Horde in 8.3 because she got her revenge which makes her and the Night Elves even with the Horde now (not really, but according to the writers).