If all you care about is the database, then yes there's a good chance that Neon will greatly outperform. But Supabase is much more than that since it includes authentication, functions, file storage, and their realtime feature. For me, the appeal is that you can get all those features with a postgres database. The nice thing is that you can mix supabase features, so if all you care about is authn, you can still use it with neon.
if you like Neon, then I imagine you like their database branching model? On Friday we announced[0] our 500K investment into OrioleDB, who are working on branching[1], with the plan to upstream these changes into Postgres core.
It would be possible for us to run a fork of Postgres today which supports branching, but our long-term view is that developers would prefer a non-forked version of Postgres (to mitigate any risk of lock-in). So we will work on adding branching to Postgres core in the background, which will be a benefit to the entire Postgres ecosystem.