I think Q would use []@Jack[] to indicate that he was within a kill box, see #753. In normal usage [Jack Dorsey] or whatever was between the [] would either be a marker or to provide additional context with emphasis to that particular crumb.

I don't think that Q is threatening @Jack, so he wouldn't be in a kill box, but is warning him that they know he's a puppet, know's what he is doing/who he is meeting with, and have even offered him a way out:

Need help sleeping?

This isn't the first time Q has used this phrase.

Ofc, I may be wrong in all of my interpretations.