The same thing had happened to me too, last night, when I was going to write a response to another member’s post.
I was actually quite surprised to see a helpful message like that, telling me that I should use the @ sign so that I could address my comments to more than one member at a time in my post.
As I write this, @christofirst might be interested in reading this as well (as I think about discobot).
If I also recall correctly, it was also telling me something like that I should reply to more than one person in my post too, instead of answering a post individually, as replying to more than one person in a post fosters more of a sense of community.
Sometimes when I use the @ key, it doesn’t always bring up the member’s name, or their name gets cut off at the bottom of my screen when the list comes up, if there’s a longer list of names of members who have been active on that particular thread that I’m replying to.