I don't think it's any deeper than Michael making the point that he's still the boss, not Vincent, Neri or baby sis, and that as boss he and he alone decides if or when someone needs to leave the planet.
I see what you're saying. One reason the quote struck me, and led to the questions I asked, was that Vincent didn't appear to actually give any "order." Not only did he get Connie and Neri's approval first, but he didn't seem to actually have anyone under his command--after all, up until shortly before he had worked for Joey Zasa. Perhaps the other gunmen were people he had hired to help him with the assassination. Or perhaps he had formed his own faction when he broke with Zasa--like the Gallo brothers in the 60s--and now did have people under his direct command.
