PostgreSQL - Base Backups and PITR
I agree that the wording is confusing.
A pg_basebackup taken with -X stream can of course be taken as a basis for point-in-time recovery.
I think the assumption here is that you take a pg_basebackup but do not have WAL archiving configured. Then the backup contains just the WAL segments required to recover to the end of the backup and reach a consistent state, but you don't have the WAL required to recover beyond that point.