It’s not hypocrisy. It is gamesmanship. If union shops are exempt, then they hope that some businesses will actively seek to become unionized. The unions are probably prepared to negotiate contracts which extend sub-$15 minimum wages for some period beyond 2022.
It is hypocrisy. They couldn’t care less about the workers they pretend to represent. They only want to take their money.
You got this totally right. I was thinking the same thing.