I aborted the interview soon after starting because of all the red flags (see below).
After applying online, got a phone call.
Red Flag #1: Never got called by the recruiter, first person I talked to was an engineer.
Red Flag #2: Launched straight into technical problem (never told me about himself, Zerostack, or asked much about me). Either poor communication skills, or hadn't used this question before, but it wasn't well explained (he kept adjusting and adding more information).
Red Flag #3: He said "did you talk with one of the co-founders?" I said no, he was the first. He replied "oh they usually call your first, otherwise they'll call after so you can talk with one." Neither co-founder ever contacted me. Clearly poorly organized.
Red Flag #4: I asked a few things about their team, and the engineer explained some of the things they were "working on improving", which were embarrassingly bad practices. I won't say what they are, but if you are considering them, definitely probe them to find out how they're doing.
At this point I crossed them off the list.
Red Flag #5: After two weeks of silence, the recruiter emails me "would you be available for an onsite interview in <two days from now>", expecting I'd be available with that little of warning, and without asking for day preference.
Red Flag #6: "The interview will be for 4 hours, from 12pm-3pm"
I would strongly recommend avoiding, they're an underdog compared to highly successful bigger companies, and clearly don't have the aptitude to compete. I expect they're just be another Silicon Valley startup not around in a few years.