Plus maybe it was first one in, last one out?
And it let Scotty wonder if they were successful just for a beat.
My belief is that they didn't change or even leave that spot, they disappeared from there and history as if they were never there. Bones interference happened before Kirk and Spock jumped in but they landed before him, so they prevented it. Things were just like they were before that all started and only the 3 of them and Scott and the others knew. I don't think anyone on the ship noticed a thing.
What I'm not sure about is, if McCoy was never there, what about Rodent? Maybe someone killed him for stealing their milk the next morning?