Yes, that's true - those products are definitely more of trying to displace the open source tech by borrowing the interface. I still think it is a plain admission to the fact of the popularity of the original tech (e.g. MySQL and Postgres). For sure, credit due where it is due for implementing entirely new implementations - but it's such a common pattern to "borrow" the open source interfaces and reimplement the core engine.

That said, you didn't mention things like RDS or Google Cloud SQL, which are cloud-based versions of standard open source dbs.