If anyone has trouble conceptualizing why Mike's speech is not credible, imagine Vecna was holding a gun to El's head and said "Mike, I won't kill her if you say you love her" and he hasn't been able to say it to her before but does now. It could be true from the bottom of his heart and maybe he would have said it regardless, but THAT being the stakes of the scene it happens in makes it NOT. CREDIBLE. AT. ALL.

To him, it seemed like it was literally he tells her he loves her or DEATH. As the ONLY TWO OPTIONS. It could have been a very easy decision because he was already gonna tell her, but if that's the case, that's not how you frame that scene.