"Where is it explicitly stated that 70% of the workforce are "actively against" the proposal?"
In the NYT and WaPo pieces I read recently. It's quite clear that Amazon workers don't want to be unionised, there's a huge, real, grassroots campaign against.
It's impossible anyone working there hasn't heard of the two opposing campaigns.
I haven't expressed my opinion here. The facts are that there is a vicious fight going on in this case, and that the minority camp is claiming getting 30% to force a vote is a victory.