Ok let me explain some things about this MU:
1) Yes Corrin has more range than Luigi BUT what are his approach options? He HAS to approach via air (as most ground moves either make him stay still or are pretty bad). When he does this, pelt him with fireballs and eventually it will lead to a good situation for us since he has no answer for it.
I mean sure, if we go on the offensive, Corrin may be able to keep us at bay for a while and give us 30% damage give or take but Luigi can dish out around 50% with one grab (and that 30% mostly comes from us being reckless for a grab).
2) Corrin's recovery is as gimpable as Fox/Falco's but made even EASIER. Just get hit by it, your down B cancels and he will fall to his death. Barely even have to try to get a kill this way, putting Corrin at a HUGE disadvantage.
3) Corrin's counter is both something good and something bad for us. If you're an inexperienced player, you're gonna get killed by this...a lot...BUT if you're experienced, you learn when is the best time to bait it out and then punish accordingly (possibly getting the kill). I barely get caught by it anymore due to practice with a friend of mine who has a Corrin secondary.
So in the end, what advantage does Corrin have over Luigi? Range and recovery length. That's it. Luigi beats him pretty much everywhere else but it doesn't mean it'll be that easy for Luigi. In the end, this is more of a 50-50 to 60-40 Luigi's favor.