Sir,
IMHO: That’s just a wise tale. I know engineers and they run the engine as they see fit, and I doubt any ever took that kind of instructions from the Conductor.
I could have added that the throttle might not had been always wide open on the helper, but at a setting that had always worked best for that engine on that specific run to fit the size of the train and location of engine placement. There was a pattern and it was always used. These guys did it day in and day out. They knew what worked without a Conductor nosing in. What if there was a gon in the train to climb down in and up out of? It did not happen that way!
That's as believable as having the fireman ordering the Conductor to shovel the coal account he was watching the water level and didn't want to blow up the engine. But it fun reading such stories.
Strange things sometime do happen, like the one time this summer the C&TS engineer took a request from the Conductor to speed up the pace and he complied, whereby, almost immediately derailed a wheel in the train. That was the last request of the season, and last of complying to that kind of request.