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.
Let me prefix this with that DHH's opinions suck, not sure what he's going through but clearly something, and that blog post you're referencing is terrible in lots of ways. But he's not openly calling for ethnic cleansing, you're doing speaking against him a disservice by not providing an accurate view of what shit thing he said. It's still terrible, the verbatim quotes (https://news.ycombinator.com/item?id=49448148), but deporting isn't the same as "ethnic cleansing". Even people we disagree with vehemently should be disagreed with honestly.
Then that blog post is just one of many where he've essentially gone of the rails in terms of hardline authoritarianism and fascist ideas, which makes it kind of weird he's getting the financial support he's getting. But then some just don't care about what viewpoints others have, I suppose that's a valid stance too even if I disagree with it.
I’m sorry, but forcibly deporting people because of their ethnic background is the very definition of ethnic cleansing.
In general, if people use violence on the population as a way to either kill or forcibly expel them, there would be a strong case to call it a genocide.