An interpretation of that clause would be that Apple either has to release Mac OS X under the GPL, or get that commercial license (I haven't checked, but it probably isn't free).

I do not think the GPL license implies that, and as another poster said, that interpretation would probably not hold in court, but why take the risk?

Also, shipping this in the past does not imply they were happy doing that. They might have found the alternatives not good enough before, just like they did with gcc/clang (I am not sure gcc is completely out yet, but it certainly is on the way out)

They would obviously not have to release OS X under the GPL, but they could be forced to release the applications using the client libraries. On the other hand, this has been true for years; this policy was created by MySQL AB, even before Sun bought them.

Perhaps Apple is interested in publishing some software linked against the client libraries.

The MySQL client libraries are GPL, which is very restrictive for a client library. I think that even the commercial vendors' licenses for client libraries are less restrictive.