Is the GPL actually viral across dynamic linking?

They're could be brought in-tree, unlike proprietary modules. There's out of tree kernel patches; that doesn't mean everything they use is external API. And many Linux systems run many modules built from the kernel source, not as external programs attached to an external API, but as kernel functionality it's useful to load only in certain cases.

I think the the court would at least acknowledge that there was an API for everyone and certain symbols that were only designed to be accessible to modules that could be part of the kernel.