Yes, natural efficient use of the body is internal.
Examples are most athletic endeavors. The body only works best in a few ways, people tend to use those ways over and over again in all physical activities.
People who believe correct fluid body use is unique to a handful of martial arts are giving internal a bad name.
No one's said that "correct fluid body use is unique to a handful of martial arts" dude
If anything Mike's pointed out that this stuff has been "common knoweldge" and a core of most MAs.
There is precedent for this stuff being used in sports. But none of it came from within the sports world. Tohei trained some baseball player and as a consequence he broke Babe Ruth's whatever whatever record. (I'm too lazy to look up the details)
I know that Ushiro Kenji has been coaching some high school basketball kids in using some of the stuff he has.
Kono Yoshinori has been making waves over here as well as a sports consultant etc.
Anyways Jin/Kokyu skills aren't necessarily exclusive to MAs, but it's most widely seen in the upper echelons of some MAs.
As for the body only "works best in a few ways" etc etc, I still think you just haven't seen enough of what's out there