The Standard Lightning Page Components lists and describes what each does. There's a couple that cover the area you're looking at, but it's not clear if there's a feed below that component on the screenshot.
Chatter Feed Component - displays the feed of the record
Chatter Publisher Component - what you have (that we can see) in your screenshot, ability to ...
That is how the "slash mention" currently works. There's this idea asking for what you're thinking it should do.
I did see this workaround suggested by a comment in the idea
Create a dedicated Chatter Group and then @tag the group instead of another record. Groups do have a related Records list
That workaround only works if that fits your use ...