A maintainer can't approve a patch that wasn't properly signed off, that's one of the very few rules set for patch submission that has nothing to do with the code but everything to do with the legal implications of including code that isn't properly attributed and an essential part in keeping the Linux kernel unencumbered.

This is not some rubberstamp formality, it is an actual declaration of authorship and shields the kernel maintainers from possible claims of copyright infringement.