If a user is tagged with a @ symbol is mentioned in a comment that they did not start or otherwise were involved in, would they get a notification?
As far as I know, these are the situations in which you will receive a notification:
If you comment under someone's post (Q or A) they will be notified regardless you used the @ tagging.
If some user commented under some post, you can then proceed to @ tag them and they will be notified.
Do note that if a user has not joined the comment thread, or it's not their post they will not be notified (again, as far as I know/recall).
This is similar to chat; you can @ tag someone in chat only if they have joined chat recently, otherwise the tagging will be ineffective (and you won't get an auto-complete suggestion).
In a few words, I think that if you don't get the auto-complete suggestion the user won't be notified, unless it's their own post.
Edit: Consider checking this link David shared from MSE that explains tagging on detail: How do comment @replies work?