Accidentally replying to a post instead of replying to a comment is a Sync bug (I think it happens if you try to reply via a Notification). I don't use it, but that app seems a bit unmaintained.
To ping a person, it's like what you did, but you need to include the instance (e.g. @[email protected] - most apps should auto-complete it once you start typing)
Voyager is probably the most popular. I like Thunder, personally (both are open-source but Thunder isn't completely FOSS because of the language it's written in).