It wasn't a myth, it was the recommendation of the
Thompson-LaGarde ballistic tests of 1904.
" Finally the Board reached the conclusion that the only safeguard at close encounters is a well-directed rapid fire from nothing less than a .45-caliber weapon."
It seems to have been less-than-scientific, in that all the rounds of less than .45" were jacketed.
It certainly directed them towards the .45 ACP round. Colonel John Thompson later went on to invent another .45 weapon that became quite famous.
I'm aware of the Thompson-LaGarde tests. I started writing about them in my post but then deleted that bit for the sake of brevity.
What I said was "myth" was that the requirement came from the need to shoot Moros. This was according to an article that I read some time ago that went back to the primary sources and could not find any line of continuity between Moros and the .45 requirement.
Instead the requirement fell out of the Thompson-LaGarde tests that you mention. Opinion seems to be however that Thompson and LaGarde started with the answer of ".45" and then constructed their tests to justify that decision.
As I mentioned in my post, there were tests conducted on livestock (live, not dead carcases). These were part of the Thompson-LaGarde tests and formed a major reason for the .45 requirement. The reason livestock were part of the tests were as proxies for horses. The reason that effect on horses was important was that the requirements were driven by the cavalry. Having a big pistol in a large calibre was traditional for the US cavalry, so in any new pistol they wanted more of the same.
The grip safety also grew out of a cavalry requirement, as they wanted to be able to holster the pistol quickly on horseback and have it made safe without having to manipulate an applied safety.
So, we're not actually in disagreement on this.