It's funny, because developers do this sort of thing all the time for much less equity.
Startup: Hey, come work here for below market salary for 5% that vests over 4 years Dev: Sounds rad!
This guy: Hey, work on your own project for 50%, and you only have to work on is the MVP for a month, and I'll pay you sort of crappy but probably better than other people in other fields. HN Devs: You. fucking. bastard.