Sweeping Strikes doesn't say you can attack an unengaged unit in melee, it effectively makes you engaged with them, so it doesn't violate the melee rule as far as I can tell.
Although Sweeping Strikes, oof...Hawthorne isn't engaged, so Sweeping Strikes treats his front edge as contacting, so if you are touching them you are engaged, so there aren't any units you aren't engaged with, so your front edge isn't treated as touching any of them, so you are no longer engaged, so Sweeping Strikes treats...
NOOOOOOOOO, I'M STUCK IN THE VORTEX
But yeah, I'm going to go with his interpretation for now, lacking anything else official. Everyone has made excellent points on both sides of the debate. The real deciding factor is that BROOKS IS THE ULTIMATE POWER IN THE UNIVERSE