Neon works at the storage level: https://neon.tech/docs/introduction/architecture-overview

So all extensions including Timescale work. Postgres does the heavy lifting

Does this affect ACID guarantees at all? Could Postgres ever expect to read back a page with currently-changing data from the Pageserver before those changes get propagated there? Will the Pageserver cluster block I/O if it’s waiting on a page to reach a consistent state?

Neon is as consistent as Postgres. B/c it's Postgres. All we did was swapping storage.