No, replying works – just the autocompletion doesn't. The autocompleter only autocompletes actual commentors. We decided to not autocomplete editors, because a) it's a very rarely used feature, b) it'd be quite some work to make this work nice performance-wise, and c) it would probably be more confusing then helpful ("Who the hell is this User12345 and why is he in my autocomplete list?").

No, replying works – just the autocompletion doesn't. The autocompleter only autocompletes actual commentors. We decided to not autocomplete editors, because 

a) it's a very rarely used feature, 
b) it'd be quite some work to make this work nice performance-wise, and
c) it would probably be more confusing then helpful ("Who the hell is this User12345 and why is he in my autocomplete list?").