Adding tags (such as [confirmed], [invalid] [resolved], etc) to the title of the thread manually will work for anyone, as that's just 'dumb' code.
(Hence why normal s can use the [web] tag themselves.)
It won't pin the thread though. Nor can normal s move the thread.
I actually asked about letting s mark their own threads as [resolved] before becoming a member of the team, as that does work properly as there is code that automatically moves the thread to the Resolved Issues sub-forum if an issue is marked as resolved. But that wasn't gonna happen.
I'm not sure how much I'm supposed to really talk about this, but basically if a staff member marks a thread as something, that gets logged in Slack, and if a 'normal' s does this, it doesn't get logged, as it was a manual edit, and not a proper tag.
All of that said, I wouldn't really consider it a bug, but it's not a 100% correct either. So yeah...
Edit: Confirmed under Low Priority.