I think Luigi is just anti grab. He is scarier up close than almost anyone at most percents with good jab, tilts, smashes and a strong grab game for damage and killing. However he has trouble getting in once again. I think as Diddy players learn their entire movesets instead of just the easy cheese strategies, Luigi will lose. His traction and range make him easy to keep out with Bair, Fair, Ftilt. His airspeed makes it hard to punish spaced aerials and Diddy can gimp him just fine.
This does remind me of Brawl's early meta where any character that could punish tornado was hailed as a Metaknight counter when it was really just MKs not using their entire movesets to their full potential. Bowser, Jigglypuff, Yoshi, Sonic, Falcon were all among the characters that at some point "countered" MK because they had unique options for dealing with 1 prevalent tactic.
In the end though, Diddy is a pretty versatile character. He has some of the fastest frame data in the game, especially in the air where he has more range than some of the sword fighters. He has quick/strong smashes, an epic command grab with built in mixups, an item projectile that can hit-confirm into kills and be used for stage control (something smash 4 diddies are doing a terrible job at so far).
He really does remind me of ICs, where he has alot of strong options, but one is so strong that its almost irrelevant how clever you could be with the others.