But they haven't made the 3D games less hard. They've made them easier to
understand, and in doing so increased the difficulty of the platforming.
I think when Miyamoto is talking about "too difficult" he's talking about how obtuse the mission structure of 64 could be to some people. The objectives in Mario 64 are super easy in terms of challenge, but interpreting the clues to figure out what to do, while also very easy, were an unnecessary barrier to entry. Having straightforward goals allows them to do more with the player because they know the player isn't getting lost.
And while having those straightforward goals, they also catered to fans of adventure-oriented goals by having sub-goals in the form of green stars and stamps hidden throughout the levels requiring many different adventure-style objectives to find them. The stars are just as required to beat the game as the stars in Mario 64. The only major difference between them is that the stars in 3D World don't force you to replay from the start of the level every time you collect one.
For example, take this level:
World 3-3: Shifty Boo Mansion
If the level title screen displayed this:
and every time you collected a star/stamp you were booted back to the world map you'd have something very similar to Mario 64.