That's not correct at all, because the plugin does not contain any WP code. It's like a Linux application which runs on GPL operating system via its API. As you should know, not every Linux application is a GPL product despite the fact that it can't run without Linux (a GPL environment).
The same with WP plugins. Here is a quote from the WP freedom declaration:
This means that all plugins and themes in the
WP repository are GPL, but it does not mean that those downloaded from other sources are GPL too. So if you have released a non-GPL plugin or theme, it won't be accepted into the official WP repository. They
do not recommend such sort of extensions, but they
can not claim that every 3rd-party WP extension is a GPL product.
This is exactly that happened to my own
Simple-FLV plugin which was removed from the WP repository because it was considered as a non-GPL one (even if I stated that it is in the license) because the package does contain a non-GPL code. I got this explanation directly from the WP team. I was also told that if I want my Simple-FLV plugin to be included
into the repository, I have to make it GPL-compatible, otherwise it just can not be distributed via WordPress.org.