Mark Twain said there are Lies, Damn Lies and Statistics.
Sure, the body has conditioned responses. Denying so would be silly since there are, as you mention, numerous studies and years to go over the results. What you're missing is the fact that well trained people tend to perform despite the onset of natural reaction because they have yet another set of conditioned responses.
Remember Ms. Assam this past year in Colorado? She didn't deny being a little scared about what was happening, she even said she had a small "meltdown" afterward. What made it happen afterward and not during was a set of conditioned responses that over-rode the simple "fight-or-flight" response.
Do you really think our soldiers manipulate weapons, think strategically and tactically on their feet and under fire and never have "episodes"? What keeps them alive is training, to include stress management (polite term for "crap yourself later, not now) to defeat the immediate stress.
You do not live at the whim of a primal defense mechanism. Mastering your own actions and responses is what seperates us from the apes. Personally, I have higher aspirations than dragging my knuckles and licking ants off a stick.
I think you understood my post perfectly, you've just not seen the other side of the argument. As an aside, the whole "slingshot because it's a gross motor skill" is a recent invention that accompanied the advent of the Glock handgun. It has no useable exterior controls other than a trigger, so an alternate method of returning it to battery was needed. Of course a supporting argument was needed, so...
The M1911 served the U.S. military for over 75 years, during which time the official doctrine was, and still is for the M9, to use the slide stop. You'd best tell our troops they're incapable of doing so before someone gets hurt.