This change is about preventing AI from trying to own the change. A human must own the change.
AI cannot own a Firefox contribution. AI cannot commit code to Firefox. Only a human may do that.
If a human uses AI (or autocomplete / a formatter / a transpiler / whatever else) to help them author code, that doesn't devolve them of responsibility. The human must take ownership and responsibility for the output.
For example, if we later run git-blame on a section of the code, we want to see the human that took responsibility for the code, not some AI.
Firefox's policy on AI code: https://firefox-source-docs.mozilla.org/contributing/ai-coding.html