Having been in two unions in a prior life, I'd say the only reason this is true is due to lack of in-person communication. Anything documentable will be kept to semi-acceptable levels. The true (daily) abuse comes when they return to the classroom. These folks need to prepare for some bullying.
Most teachers defying the union on this will not make it more than another year in that district once classrooms return would be my uninformed bet. It will be a mission of every other union member at each school to make their everyday existence a living hell.
Yes, I have very poor taste in my mouth when it comes to my experiences with unions. I certainly recognize what they've accomplished and could still accomplish; but until they stop existing as corrupt rackets to protect the lowest common denominator employee they are going to be a hard sell to much of the US who has dealt with such creatures.
Union members automatically pay dues. They have the option of paying more dues. Someone who paid the automatic dues went to the union for help. Each time, they were urged to opt in to paying more dues. They chose not to, and were left waiting for help. When they finally chose to increase their dues, the help suddenly materialized.
And sure, most speech surrounding the bullying isn't direct. Would you expect it to be explicit? That simply isn't how any remotely intelligent person makes illegal threats. But it's pretty easy to pick up on the tone of "hey it's a nice job you have here. It would be a shame if something were to happen to it"
I support unions, I think they provide a net benefit to workers, but power structures frequently attract people more interested in wielding the power than in the purpose that power is suppose to serve. I see too much of a tendency in supporters of unions to overlook this fact, with any criticism dismissed as "you don't support the workers!". (Note: I'm not accusing you of that. We appear to be having a reasonable discussion)