In my comments to Can (quantum) angular momentum $L$ be zero? I tried to @PPG, but it won't let me. Presumably the conditions for whether or not I can ping someone are listed somewhere ...
The rather lengthy conditions are given on Meta.SO: How do comment @replies work? In particular, these two points explain that
@-pinging is both unnecessary and auto-deleted in your case - both you and the author of that post will ping each other automatically:
If a user comments on their own post and there is only one person who has previously commented, then that person is also notified if @name is forgotten.
The first author of the question or answer will always be notified of any new comment. There is no need to use @name to notify them. (You may still use it for clarity, if needed; however if only you and the author have been commenting on the post so far, the @name will be automatically removed from the beginning of the comment, as it adds no value.)
The owner of a post is always pinged, so you don't need to explicitly mention it (and thus autocomplete doesn't work).
For that matter, editors of a post, closing moderators (not 3k users), and bounty-givers can also be pinged, though they don't turn up in the autocomplete.