Oooh, that. Have seen this notion about ENFPs plenty of times over the internet. To make it short: I think the chances are high that ENFPs are the most introverted extroverts as they have Ne and Fi. Ne is a function that doesn't need to be played out loud, so whatever Ne comes up with, it's in the ENFPs minds, so not heard or seen by anyone else. Unless they think they just had a flash of genius, most of what Ne tells an Ne-dom will never be heard by other people.
Mind you, my experience is though, the younger the Ne-dom, the more they have the need to tell you about the treasures Ne unburied. Will get to that later.
Then of course there's Fi, which makes ENFP need more down-time to reflect and/or digest certain feelings, experiences, and also what makes ENFPs do lonesome activities, like drawing, reading/writing a book, watching a movie ...
So that combination of Ne+Fi makes ENFPs at times appear more introverted.
That chimnes with the experiences I made with other ENFPs. There are many factors which determine how extroverted an ENFP is. The younger an ENFP is, the more outgoing - it may take an ENFP a while to understand that most people they come across don't use Ne and don't really know what to do with your Ne-findings. But with time comes the experience to know when an Ne-finding is worth discussing, but I am sure the older an ENFP gets, the less those findings really need to be talked about, for a huge lot of different reasons I won't get into.
Often times it's also a matter of Enneagram type: a type 4 may have more access to Fi, thus using more Fi, being more introverted, you get the gist ...
But even when an ENFP isn't a type 4, they still have enough Fi to act introverted quite often.
Last but not least, when it comes to myself, the times I percieve myself to be extroverted rather than introverted is roughly %60. That's not a lot for an extrovert.