Those are the reasons why I think it was more about Metatron back there and not Gilgamesh, because of how they are lifting up Metatron around that story and making a few things clear. Also explaining Zachariels part of it all. aside from that, note that both Metatron and Zachariel are always smiling through that meeting.
Now to see it from Gilgamesh’s story
Dantalion says Gilgamesh was summoned by God and joined in with the Archangels in the fight. Archangels, not Michael. The only time Michael is mentioned is when comparing who is better to serve under, Lucifer or Michael. The two being the official leader of each side. That is, until Gilgamesh says that the real leader isn’t Michael, but Gabriel. It was never ‘only Michael’ that was betrayed but heaven itself. Michael even says ‘us’ instead of me, someone that self centered as him could easily make it personal if he had to.