Making a single comment during a debate, then backing it up with a source, is "indoctrination" much in the same way that chopping up some firewood in my backyard is "deforestation".
It wasn't a single comment. It says so right in the OP.
He'd been warned at least a couple of times previously about parental complaints. This was just the final straw, one he described as "will probably get me fired", according to the story.
I don't think he wanted the job any more.
Being so far out from the parents made it uncomfortable. But the parents are ultimately in charge of their children, for better or worse. It doesn't matter if he closely aligns with your world views, you aren't in charge. Neither is the school administration, they had to choose between keeping their jobs and Hawn keeping his, probably.
Let me ask, who do you think will teach that class better? Hawn, who "can't help himself"(according to the report), or his replacement? A replacement who will probably be chosen for their ability to avoid any controversy that might further annoy parents who are already angry.
Did Hawn do those kids a favor by showing them the YouTube he chose? I don't think so.
Tom