If you're below them? You can't DI up. What I mean is staying out of their zone where they can't hit you and being below them. Also, if you try to hit them with the Nair, you usually would have to hit them after they land, and it's difficult to time before they throw out their shield. If you don't try to hit them when they land, and if they're too high above you, and you only hit them with the top section of the Nair, it's a lot easier for them to DI out of it before they're set up for the Up Smash.
Like I said, neither of us are "wrong", we're just talking about two different situations. While what you're saying does work, and is definitely effective, it doesn't apply to this situation.
Edit: Guys I'm in the Oli Chaaaaatttt!!