Do you mean how to add the reply functionality (e.g. tag user, emoji and etc) into the Easydiscuss comment section? (screenshot: https://monosnap.com/file/k6srKAh2mXs9dgXC0VBGYClY0BNfQz )

If yes, unfortunately, there is no way to do this at this point of time.

Yes. That is what I'm referring to. ...If the editor for comments was the same as the editor for replies, would that provide the extra functionality? Or are there additional processes going that handles mentions (@someone) so that a mentioned person receives a notificaton that he was mentioned?

Yes, if render the BBCode editor in the comment form then you should have those user mention functionality.

But I can't guarantee 100% that will work correctly if straight away render the BBCode editor in the comment form without any testing.

Even if you replace to use BBcode editor in the comment form, you also need to check whether the following things work correctly or not and need to customize some of the core files as well for example:

1. Ensure that the comment content stored in this table #__discuss_comments

2. Ensure the comment content is parsed correctly for the emoji and the user mention code in the comment listing similar to how the system handles the reply content.