Yes, but charismatic people are so natural in their expression/personae that nothing they do or say ever seems *forced*, so any self-control they do utilize/exercise is deftly quick and hence, unnoticeable.
For some charismatic people, it can be almost effortless, that's probably right, but I think a lot take steps to overcome or control certain aspects of their personality that conflicts with being charismatic. Anger issues, self-esteem issues... any disorder or negative really. No one's perfect.
I dunno, perhaps I am seeing this from an ENFP standpoint, but your description of a charismatic person as using, "The well-placed word, knowing when to say what, and how" seems, *forced* and Machiavellian to me, and therefore indicative of a snake-like charm and not a natural form of charisma.
I could be wrong though.
Charm vs. charisma vs. machiavellianism....
The difference is merely what comes naturally, isn't it, as you said; and I've said up there that the person is always at ease with themselves...
and how suspicious the purveyor is sometimes matters too.
Someone who naturally puts others at ease, it could be learnt from observation that it is what the other needs sometimes, so you give it. Is that machiavellian... knowing how to respond in a way others want.
I know the snakes you mean, I face them at work. FWIW, anyone with charisma wouldn't be a turn-off nor leave an oily feel.
The difference with an ENTP/INTP especially in mature ones would be less obvious, I think.
Mainly because the older INTP learns that imperfection is ok, and becomes very chill. That's the stage they're lovely to be around with. I think they could resemble ENTPs there in the willingness to engage in Ne type ideas and explorations, "just to see".
With ENTPs, there's a greater awareness of self, and that provides the grounding to the flightiness they may have exhibited in earlier ages / bouncing around uncommitted to anyone and anything.
FWIW, I find the sympatico with INTPs something harder to find with other types. But I could be biased since I grew up with one.