I find this interesting, and maybe I am just a bit confused. But here is what I am wondering now.

If it is not about the use of an API but about the use of the implementation, then that would mean that the end-user that is running the application which dynamically links to a proprietary library on the end-users pc the one violating the license terms, not the developer that distributed their code without distributing the proprietary library itself.

And if the developer has the rights to distribute the library, because it is under GPL, then they could do so, and would not violate the license terms, because they are not doing the dynamic linking and using of the implementation, that would happen only when the end-user starts the application.

I never heard about any case against end-users, that for instance used the proprietary Nvidia kernel module, compiled and linked on each end-user's PC.

I don't think that works that way, there has to be more to that than just the usage of the implementation that triggers the license violation.