This sounds like a great opportunity to define Internal Strength terms in aiki speak and vice versa. Would be a great opportunity to have a reference post or "sticky thread".
As I've noted elsewhere, the term "internal strength" comes almost directly from "neijin". Of course there are numerous ways to train "internally", hence a term like "Neigong" ("internal exercise"). But neigongs are really a subset of "qigongs" ("qi-developing exercises"), which you'll hear sometimes in Japanese as "Kiko".
"Kokyu" seems to be a more widely used term than just in Aikido because there are a few different other Japanese styles now offering videos on "Kokyu" and it seems doubtful that a number of unrelated styles got their information from Ueshiba, Takeda, etc. Ushiro Sensei seemed to use the term "Kokyu" often in as general a way as "Aiki" is being used by some people today.
BTW, internal strength is shown/demonstrated in a number of generally accepted ways in China. I listed them at least twice in posts in the past.