Consider an arbitrary piece of code that you wish to do something with, such as copy it, run it, modify and distribute changes, make add-ons for it, etc.
If there is no copyright on the code, and it is not covered by patents, you can do all those things without having to get anyone's permission.
I'm going to ignore patents for the rest of this comment. Most code is copyrighted. For copyrighted code, the law says that the copyright owner has certain exclusive rights. The ones usually relevant to software are to make copies, to distribute copies, and to make and distribute derivative works.
If you are not the copyright owner, you can't do these things legally unless you have the owner's permission.
The WordPress copyright owners have given permission to anyone who follows the GPL to do some of those things that copyright law says require their permission. If you haven't otherwise arranged to have permission from them, then the only thing that gives you permission is their blanket grant of permission to everyone who follows the GPL, and so the GPL is indeed an important factor in what you can do with WordPress.
If they added something to their T&C that said themes must be GPL, and you wished to write a theme and in writing that theme you were going to do things that required WordPress's permission under copyright law, then yes, your theme would have to be GPL.
The issue, though, is whether writing WordPress themes necessarily involves doing things that require permission under copyright law. If themes necessarily are derivative works or copy from WordPress, then the WordPress license (GPL) and any other terms and conditions must be obeyed.
If a theme does not copy code from WordPress and is not a derivative work of WordPress, then the theme author doesn't need permission, and so the license WordPress uses (GPL) and any other terms and conditions they have are irrelevant to that theme.
That is why the definition of "derivative work" under copyright law is so central to this issue.
