They never said "There will be no loading screens whatsoever". Also, NML+Novigrad, and Skellige maps, by themselves, are larger than Skyrim's map. As a person who spent 80-90 hours in DA:I, I can say that those maps don't even start comparing with these ones. DA:I had several big maps. Witcher 3 has two open worlds, and also several big maps. Also, remember when you try to go to the other parts of the castle in DA:I you had loading screens? They don't exist in Witcher 3. You see a door, you enter. That simple. That's what "seamless" is. (There is even no loading screen when you go inside the Sewers of Novigrad; which by itself nearly spans the whole city. No loading when you get inside a house - with more than 100 houses to get inside in just Novigrad. No loading screens when you go inside a dungeon. And so on.)
Normally it would take at least hours to sail to Skellige from Novigrad. They didn't want to squish the two landmasses together so that you'll be able to travel between them in minutes (Remember how you could go to another city in Skyrim in 5 minutes?), so they put a loading screen between two huge maps.
I seriously can't understand how the two games compare in this regard, but that's just me. I'm sorry for the aggressive tone in this message, by the way.
Note: Sorry mods for discussing between DA:I and TW3, but the argument required that.