As described in the OP, I am 100% calling that out. @ang2bmd nailed it. The pitcher's reaction after the violation is inconsequential. Many newer umpires will miss this call because they are placing improper weight on the pitcher's reaction. Reaction ... not action.
As @marriard noted though, often the eyewitness account and the actual event can be different so this is, as we say, HTBT ("had to be there" for our cool new members!).
Thanks for catching this @The Man In Blue as the original post says the runner did change their direction after they stopped and was committed to go back to first and that would be an out from the look back rule...