That's true. However, if you don't already have any "traditional" (binlog-based) replicas on Aurora, you might not even need gh-ost or pt-osc in the first place -- at least for many common ALTER operations.
Aurora supports physical replication at the storage level, which means you can use MySQL online DDL (ALGORITHM=INPLACE, LOCK=NONE) without having to worry about replication lag. Recent blog post with findings from Percona: https://www.percona.com/blog/zero-impact-on-index-creation-w...
Caveats:
* This is safe on Aurora 3 due to MySQL 8's atomic DDL support. On previous versions, there may be crash-safety risks, which make external tools safer for altering very large tables.
* External OSC tools can provide other advantages, such as throttling, safer to cancel, ability to time the final table switchover, ability to rollback (pt-osc supports "reverse" triggers), etc.
* Some forms of ALTER TABLE don't support online DDL, and for those you'd still need an external OSC tool like gh-ost or pt-osc, but it's less common.
