This is a reference to a dog whistle. Above 25kHz are sounds that dogs can hear but humans generally can't.
The term dog whistle is used to refer to phrases that racists use to indicate their racism, while sounding plausibly deniable. The idea is that it's a phrase that will make other racists realize that you are one of them, while not sounding openly racist to others.
DHH himself has gone beyond dog whistles. He's directly compared what he considers undesirable ehtnic populations to wolves who need to be shot. He's just openly calling for ethnic cleansing by now.
But support for Omarchy itself can be seen as a dog whistle. It's widely known that DHH is a fascist, but you have some plausible deniability by donating to Omarchy because you're just politically neutral and think it's cool tech, or something like that.