This is a retarded point. Sylvanas could have easily let Varian die in the Legion cinematic but instead ran in to help him. Ditto, her staying there would not have saved anyone. Even BTS that has as its main deal to throw her under the bus explicitly has Baine and Anduin, chief author's mouthpieces who the narrative never casts as doing anything wrong realizing this and going on about how it was the right thing to do. Trusting Sylvanas there was the correct call and BTS goes on and on to even show us how Sylvanas felt terribly bad about it in her own mind but it was the only option.
Now everything in BTS re: Sylvanas's internal monologue is essentially non-canon given how it had fuck all to do with her goals in both BFA and her slightly different goals in Shadowlands, but the point is that when the scene was written, Anduin was meant to be correct about trusting her and even retroactively, the alternative is worse. It's not a case of him being flawed. Nothing Varian could've done in that situation re: Sylvanas would've saved him and no one ever calls Anduin on this anyway.