I’m not arguing with him on the status of the mikvahs; I don’t care whether or not he uses him.
The point is, he’s talking to people who got p’saks from their rabbis not to use Chabad mikvahs. What would you like us to do, posken like CR User: Milhouse instead of our rabbis?
I can’t speak to whether or not “all shittahs” are against Chabad Mikvahs. But, his assertion that nobody says they’re posul just isn’t true.