It doesn't need to be spelled out. it needs to be addressed in some manner. This is my problem with you, you see everything as just a dichotomy and it isn't. It is either X or not X, and that isn't how things work.
Just because the threat is known to the audience does not mean the threat is know to the characters (that is actually evidence of shitty writing) ... well, then they did shit writing because a character that saw Sylvanas kill Saurfang was arguing for caution, meaning it is clear even someone close to Tyrande doesn't see Sylvanas as a current threat worth devoting resources too. And look at that, we can see from ONE LINE that a character realize that devoting resources to Sylvanas is a bad idea because they view her as a reduced threat ... and guess what? IT WASN'T SPELLED OUT. But, totally, couldn't remotely do anything for that with Tyrande in 5 lines ... that's too few.
The problem is they have nuggets of decent writing, but are shit all around it.
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Literally, not.
You could justify in your writing Tyrande's irrationality by addressing and referencing directly in her line other aspect.
They don't need to be robots, but they need to be more than "Oh, my enemy wants me to do this ... I know, I'll do exactly that!" They never even addressed that Tyrande KNEW she was playing into Sylvanas' design ... she was just "Screw it, I am taking back Darkshore. I don't even care/remember that I know this is Sylvanas' plan." It is stupid shitty writing.
So, any complaints you have about the writing are literally null and void. To me, you are just a whiny hypocrite because if you didn't have the shit writing you would have nothing to whine about.