I can’t claim full understanding, but what I took away from it was that NVIDIA somehow ended up using GPL-licensed code in their proprietary drivers, possibly in a way that could incriminate the Linux kernel if not handled properly. My best guess (as someone with no kernel programming experience) is that NVIDIA sometimes contributes code directly to the Linux kernel that exists solely to support their proprietary drivers (the shims mentioned in the article). Apparently, these shims were exporting GPL-licensed code for use inside the proprietary drivers, which would be a violation of the GPL (unless NVIDIA made the source code for their proprietary drivers freely available in compliance with the GPL).

TLDR: (I think?) NVIDIA essentially infected the linux kernel with license violations to support their proprietary drivers, and the linux kernel devs are working to excise the violations and prevent anything like that going forward.