That is somewhat of a concern, but less so now that they support web services. So, you would still need a basic oracle DB set up in order to run APEX, but you could just use OracleXE, which is a quite capable free version, and then connect to whatever you want.

Also, you see this argument a lot - "what if I want to switch databases"? I've seen more than my share of overly-complicated and highly non-performant code bases, "just in case we want to change our database at some point" (see the ORM messes out there.) Its a problem in theory, and in my experience, not in practice. Never in 25 years of IT work have I switched databases, so it's often a classic case of "prevention worse than the disease".

One case where this use to be an issue was for software vendors that used to sell applications requiring a database, and they had to be ready to work with whatever the customer had. In our day of cloud based apps, this is increasingly becoming less of an issue.