Looks like it's explained here: https://github.com/charlotte-os/Catten/blob/main/License/cla...

Specifically, "Users may link this kernel with closed-source binary drivers, including static libraries, for personal, internal, or evaluation use without being required to disclose the source code of the proprietary driver.".

There's a note in the repo that clarifies the meaning of the GPLv3 regarding the use of combining covered works with proprietary libraries when the resulting combined work is never conveyed. It doesn't modify the license, it just explains what it means in that specific case as we interpret it.

Also to be clear I am not a lawyer and nothing I say constitutes any form of legal advice.