@Sphinx
That is a good question, I see more experienced users consistently doing that, and try to do the same myself. Like on this reply.
Harder when there are more people mentioned, and specially when the person you meant to reply to is at the very end of the list.
That would require a cut / past operation on the toot editor, and takes a bit of time, so sometimes I just leave it the way it is if there's lots of posts to reply to.
@design_RG
Yes, i just clicked the mouse and input the text between two person.
@freemo