Specifically they seem to violate: Content-Type: application/vnd.api+json without any media type parameters.
and Accept header MUST specify the media type there at least once without any media type parameters.
The text on the extensions page is experimental and non-normative (as it notes at the top). In fact, the extension system is likely to be dramatically overhauled soon.
So, basically, the contradiction you point out means: it’s not compliant to use any media type parameters at the moment, but, were the experimental extension to be adopted, the base spec would be amended to allow the parameters that system needs.
In concrete terms, I’d suggest avoiding the extensions for now because, as I mentioned, they’re likely to change dramatically.