"And you know what that means" means, in context, "All the bad things about them are true and they have no arguments to support their point of view."

That's the claim he's making. At least, that's the standard claim when people argue that their opponents are demonizing rather than discussing. It's not a dogwhistle--it's just something that gets said enough that it doesn't bear repeating.

(Pretty ironic coda, given that he is painting doctors who cover gender-affirming care as a threat to child safety, and was basically saying that protestors are terrorists, and can't manage to find a fact to back up his position. Reminds me of Doctorow's recent post "takes one to know one": https://doctorow.medium.com/takes-one-to-know-one-104d7d749408)