The main point is that this is a fairly unambiguous message to other managers, that allowing unionization to happen under their watch would be a careericidal offense.
That means that they can expect the warehouse managers to get pretty desperate, and probably break all kinds of laws and whatnot. It's just that the message is being sent in a way that allows the company to keep their hands clean, from the almost certain results.
The managers will certainly get fired, if unionization happens, and might get fired, if they get caught, spiking the trees.
Guess which path the managers are likely to take?
Walmart laid out the playbook that profits in a thin margin market require you being an asshole to your counterparties, but blackmailing them into still working with you via scale.
It would seem to make sense for all the warehouse managers themselves to form their own union.