You can in desktop version, I've done it before. But it depends on how you arrive to the comment I think. If you arrive from the story like I did here, it's not possible.

But if you follow the notification or from the mail, I think it's different.

Or maybe it's an intermittent bug that allows tags in desktop sometimes! :D

I just added tags here by following this route:
1. https://medium.com/me/stories/drafts

2. Responses

3. edit