I guess database as a service is not as bullet proof of a business model as one thinks, same sort of acquisition happened with Neon.
Neon is just Postgres for the most part -- sure if they shut down it'd be a pain to migrate but you'd mostly be fine just swapping to AWS RDS/Aurora or the plethora of other Postgres providers.
Buying into things like Gel or now defunct RethinkDB mean if the primary company went under you're stuck with a dead DB engine and potentially no managed hosting options.
By default, it does start a managed Postgres because that's more convenient, but you can point it to Aurora or RDS or any other pg deployment.