What is a carbon lug?
Just like steel lugs for traditional brazed construction - except these are bonded together instead of brazed.
Colnago has made (may still make, too lazy to look) frames using carbon tubes bonded to carbon lugs.
Early carbon frames had carbon tubes bonded to aluminum lugs - my first Trek MTB was an aluminum frame done the same way.
Most don't, though, monocoque construction is the norm.