Well if you DI-in in that situation, you really would be stupid.

DI out doesn't always stop it from being a true combo. If it does, just read what your opponent does. It's pretty simple when it's either airdodge or jump. Marth has too much reach for most dair hitboxes and such. The ones that could potentially out range marth's move such as cloud have too much start-up in their dair anyway.

So long as the person is not completely stupid, it shouldn't be too hard to read what the opponent wants to do. It's the whole reason why DI-in is a viable mixup.