.. do we, then, label those arts where only a few were trained in the secrets, which dantien-controlled whole body power was most likely one? Do we then say that neijia is only a term for certain people, rather than the overall art?
Well, as far as labels matter this is tricky isnt it? If you consider that in the "neijia" this indoor stuff seems to be the norm rather than the exception. I dont know, it seems safer to let the label stay with the people who coined it and set store on it. The skill, the whole skill and the rabbit hole of how much is available to you seems to be a different level of conversation. Steal this technique is not going to be very conducive to generating a common understanding , a consistent one, or an unchanging one.
O sensei was continuously learning. He adapted things from other arts (lets not spin off into the old saw about Aikido not coming from DR). Wer'e trying to pin something that was being cooked in a long simmer to a fixed formula; a recipe that was always the same and never changed. I think that just confuses things.