388h is indeed the original adlib base port. Most sound cards that feature an OPL chip will also monitor reads/writes to this port for backward compatibility with older software, but the FM chip is also addressable from a base port offset.

Incidentally, the DSP isn't actually at 220h, it's at 22a/22ch. How the ports are mapped exactly depends on which sound blaster model you have. What's actually at 220h on older cards is the old CMS chips, while the OPL2 is at 228/229h. As CMS chips fell out of use and later cards featured dual OPL2 or an OPL3 chip, 220h-223h were repurposed for FM writes also, which means you can access the OPL chip from a grand total of 3 different IO ports.

Interestingly, cards with dual OPL2 chips would often be designed such that writes to 388h would actually go to both FM chips instead of just one, so that you still get proper mono sound, otherwise it would be panned hard left.