It looks like the autocomplete logic on bugzilla is using the list of people in committers.py. It seems like it would make more sense to query the list of people with accounts in Bugzilla, maybe?
I also noticed this issue on several occasions, and was wondering is this could be improved. I can see two problems with completing using the full list: 1) There will be too many variants, forcing you to type more. 2) Some people have multiple accounts - a huge benefit of the autocompletion feature is that you wouldn't accidentally CC a wrong clone. Perhaps committers could just add their favorite non-committers to the list manually?