This is hilarious. You couldn't get any more on-brand for Hacker News without suggesting that someone rewrite the kernel in Rust.
The people at this rally almost certainly supported Trump, and at least some of them supported fascism. As such, a slogan condemning Trump alongside fascists is totally reasonable, even if it wouldn't necessarily be in other contexts. (And if you think that someone carrying a nazi flag doesn't really believe the message, well, the burden of proof is on you).
Politics, by the way, is a tribal game. Almost everyone develops their beliefs primarily based on those of the people in their community or communities. For example, the belief that "holding an anti-Trump sign is just as bad as holding a Nazi flag, because both of them passionately express opinions" is basically non-existent outside of the Hacker News-reading demographic, but one which I've seen expressed here fairly often. That's not because HN readers are smarter than everyone else: it's because people tend towards the opinions that they're exposed to in their communities, and the rules of Hacker News effectively mandate impassioned centrism, so that's the ideology of the Hacker News tribe. If you think you're immune to tribalism, you're wrong, in the same way that if you think you're immune to confirmation bias, you're wrong.
Visibility is much more important to influencing people than good arguments or sincere intentions. For example, five years ago, I was aware of the arguments for open borders, but I didn't publicly advocate for them, nor did I have a particularly strong opinion on the matter. That's because back then, there was very little public discussion of the issue. Now that I've met other people who are pushing for open borders (probably in response to Trump), I consider it to be one of the most important issues that I care about. That's just how humans work. That's the reason that people advocate for their views in highly visible ways, like protests. The point isn't to convince your enemies, it's to bring out the people on your side.
John, who derives all his beliefs from careful research and pure reason, gets just as many votes as Bob, who likes Trump because he triggers the libtards. Actually, I suspect that Bob will get more, because half the time, people like John don't actually vote. You can talk as if you're above the rabble all you like, but in Democracy, it's their opinions that matter, and if you take politics seriously at all, it's them that you have to reach. Going out in the real world carrying signs, shouting, blocking traffic, throwing bricks, and maybe getting arrested is the best known way to get your message out, even if it offends your sensibilities.