If you read the article, it is indeed full Linux because the 4004 is running a MIPS emulator that provides the necessary memory management features. Pretty much all of the “run Linux on some old chip incapable of running Linux” projects achieve it via emulating a more featured architecture that Linux supports, not by somehow compiling Linux to natively run on a 4 bit, MMU-less architecture.

It’s also a “don’t allow third party proprietary shit into your kernel” issue. If the driver was open source it would actually go through a public code review and the issue would be more likely to get caught. Even if it did slip through people would publically have a fix by now with all the eyes on the code. It also wouldn’t get pushed to everyone simultaneously under the control of a single company, it would get tested and packaged by distributions before making it to end users.