I think we innovate on different parts of the stack. Neon is offering separation of storage and compute and serverless. Theoretically you can run Supabase on top of Neon
To elaborate Neon is designed deliberately to plug into the Postgres ecosystem. So all of the ecosystem works out of the box. Supabase seems to be vertically integrated. But Paul should comment on that
I believe most of the tools that Supabase use would work with Neon, with the exception of Realtime. As I understand it, Neon doesn't support Logical Decoding yet[0], which is required so that the Realtime engine can receive updates from the database.