postgres standby - what not WAL records can be replayed?
There are lots of transaction log records other than commits.

Transaction ID allocations (roughly like a BEGIN delayed until first actual write). Inserts, updates, deletes, vacuum, lots of internal operations.

Synchronous streaming replication  only waits for confirmation that the replica has replayed commit transaction log records. Not any of the others.

Some things, like the UNLOGGED tables you refer to, do not generate any transaction logs at all. So they cannot affect replicas. But that is not what that passage in the docs is talking about.