If you take a look at the licensing tag you'll see many (22) questions about the GPL and WordPress. Way to many of them have accepted answers which are incorrect.
Many of those questions/answers were considered correct at the time. Keep in mind, the GPL is an incredibly complex but not-very-well explained piece of legalese, so every statement we make here, on personal blogs, on podcasts, or in chat rooms is our own interpretation based on past experience and what we've been told by the license authors and original adopters.
Licensing has been such a hot issue in the community because there are so many licenses out there and so many differing opinions on what is/isn't allowed. Particularly when you get to derivative works.
However, this site lends itself well to being updated. Commenting on an outdated answer asking for an update is fine and helpful. What would be even more helpful would be earning reputation by answering other questions and then editing inaccurate posts.
Simply put questions are answered to the best knowledge of those answering. Licensing being convoluted and nasty topic - that knowledge tends to be far from perfect.
Sadly situation with licensing around WP was not resolved to the "this is definitive legal explanation" or "we all agree and happy".
It was resolved to "if you voice unpopular opinion you [and your business] will eat shit from community".
So yes - there probably are some questions with poor answers about licensing. Not that much of them (in scope of many thousands of total question site has by now).
What can be done? Well - everything that can always be done: comment (wisely), edit (when really needed) and out-answer (in most awesome fashion) incorrect stuff.
This is a (potentially inflammatory) subset of this question: What to do with wrong answers marked as solution?
I would follow the advice/answers from that question. Regardless of why there are so many incorrect answers to licensing questions, in the end the real problem is dilution of the correct answer - and the solution to that problem is to use the site mechanics to downvote incorrect answers, upvote correct answers, and leave comments as appropriate.
Answer: Licensing is difficult and there is much FUD around the issue. After years of hashing this out, it's fairly well understood, but old entries still exist and haven't been updated.
Some points to clarify:
WordPress is licensed "GPLv2 or later" explicitly. If it lacked the "v2 or later" part, the text of the actual license states that the licensing terms can be upgraded to any later version.
The plugin page has some language about GPLv2 only. This is an arbitrary rule resulting from what was likely originally a typo or a misunderstanding of the Core's license (but we may never know, and it's irrelevant anyway). This deals with hosting, not legitimacy. GPLv3 plugins and themes are legitimate.
The incompatibility between GPLv3 and GPLv2+ or later is one-directional. (GPLv2 only can't be upgrade.)
You can't combine GPLv3 code into a GPLv2+ distribution.
GPLv3 compatible code, meaning AGPLv3 and Apache 2.0 among others, can be combined into legally distributed plugins and themes.
Edited because Rarst corrected me.