Guess I could test this on a live system but what happens if you post a topic and then edit it to include some more @usernames? Does the system send the notifications just to the new username added?
The answer to your question is yes.
If you (or a moderator) edit your post later to add more @ mentions, those people (or groups) will indeed be notified (assuming they have access to the post).
This is a common question in our community. We use it all the time.
Ah, I misread the question, sorry! I saw “tags” and missed the “@”. Yes, edits that add “mentions” will cause notifications - sorry for any confusion I may have caused.
This is one of the reasons I Discourse so much. What attention to detail. This is super-useful, and delightfully surprising.