@Punit S , I am finding that the suggestion partially works when implemented in an Update Record action in a flow.

var addlComments = "@<" + fd_data.trigger.current.caller_id.user_name + ">";
addlComments += ", your request is being processed."
return addlComments;

The result is that the literal  string with the < and > appears and ServiceNow does not recognize the entry as an @mention. Is there something wrong with my syntax or is this not possible within a flow?

I had to use the [code] tags technique that is documented in this article to insert the styled text.

var addlComments = "[code]@<" + fd_data.trigger.current.caller_id.user_name + ">"
addlComments += " , your request is being <b style=\"color:red\">processed</b>.[/code]"
return addlComments;