In the linked article it says:

"Unlike AWS Aurora we decided to open source all the changes in Postgres and also send them upstream as well as fully open source our cloud native storage."

Also, at the top right there's a link to the GitHub repo: https://github.com/neondatabase/neon

You can find replies from folks in this thread as well discussing self hosted deployments and you can find an entire discord channel dedicated to that topic here: https://discord.gg/92vNTzKDGp

I'm not associated with Neon and I've never used it before and I certainly don't disagree with your sentiment, but it seems like Neon has gone above and beyond in making sure that workloads could continue to run in the event the company fails and broadly assuaging those concerns.