ENFPs do seem more introverted than most extroverts.
But more introverted than ISFPs, ISTjs, and ISTPs? I doubt it.
Also, yeah sure N types may need less sensory stimulus but at what point does it start to drain them? Isn't that what matters with introverstion/extroversion? My ENFP brother may not need much sensory stimulus but he seeks it way more than I do. I get drained much faster around people and heavy duty sensory stimulus (concerts, etc.)
ETA: Just to make it clear, I don't think that just because it's like that with me and my brother that all ENFPs are more extroverted than all introverted sensors, I was just using it as one example to go along with my hunch.