Maybe so, but proving without any possible doubt that was the cause is the burden of the fired employee, who also likely doesnt have money for a lawyer, let alone court fees to persue the case in the first place.
This difficulty is only amplified in at-will employment areas where your boss can fire you on the spot for no reason at all.
Its as if the system was built from the ground up to benefit businesses over people.
So if the employee was fired on the spot, that would pretty much guarantee a win in most states unless the boss could show that they had already planned to fire them before they shouted "unionize".
If the boss fired them a month later, that would be a different story. But yeah, the rest of that is valid.