This is such a great post.

It illustrates 2 things well:

1. Function/API calls don't require license inheritance
2. Non-compiled code is pretty much impossible to GPL.

To illustrate my second point:
WordPress uses PHP arrays, and loops over those arrays to produce an output. Because WordPress is GPL, and uses arrays and loops, every use of an array and loop must be GPL.

FALSE.

Hopefully, you see my point.

But, as the article said, the WordPress project (being a private org) is free to take any action that may discourage the use of non-GPL themes, such as pulling them from the directory, publicly denouncing them, etc. So it's a risk vs. reward scenario.

Nathan