I don't buy it. The same factors that made Zoom easy to adopt, make it equally easy to switch to something else. And "when Zoom goes down, teachers can’t teach"? Come on. I am a college professor. In my class this fall I met my students on Blackboard on Day 1 and Zoom on Day 2. If one of them goes down for the day, we'll switch to the other.
I think it's for real. Think about elementary school instead. The kids are just learning to use computers. Their parents may be also, and if not they may be trying to do their own work from home while the kids are in class. There are many low-income and/or ESL families. Even the teachers aren't tech experts.
In my son's kindergarten class, everyone got school-supplied Chromebooks. They all have Google accounts for Google Classroom. That probably means they could switch to Google Hangouts Meet, but it wouldn't be smooth. They'd at least miss a day, and subsequent days would be rocky. The teacher has invested a lot of effort just in teaching the kids how to mute and unmute themselves without waiting for their grown-up to help, and they'd have to go through that again with a different UI.