My wife teaches 3rd grade and it was way more than 8 hours per week for her. For every student zoom call, there were at least two teacher-only prep zooms so they would be on the same page for the "live" class. And usually an all-faculty call at least a couple times a week. The brief videos you guys are mentioning can easily take an hour to knock out.
But I would say the majority of her time was researching what was working elsewhere in terms of methods, videos, assignments, everything. She could do that well into the night. For every little graphic, video or song that was part of the class, there are a bunch of others teachers have screened in deciding which to use.
For the record, she fucking hated the whole thing and agrees it is barely helping the kids. Her solace was that things like Katrina have knocked kids off schedule for long periods before, and society eventually recovers.