"Correct, theaters at the time could not really been project anything other than 24fps. So there were 2 parts to the shooting style, and one of them is what you describe, shooting at the higher frame rate used in order to have it play back slower when conformed to 24. But they did this in a pretty unusual way. During the action sequences, they would ramp up frame rate from 24 to 30 and back down. They would do adjust during the shot, so the action scenes had these subtle but constantly-occurring increases and decreases in speed that looked very interesting and had not been done before."

Wouldn't this just lead to judder? Maybe that was something that had not been seen before in a movie shown at cinemas.

I've heard that sometimes scenes like kung fu fights were filmed in a lower frame rate (maybe 20 fps) and then it got faster when it was projected in 24 fps. If you do it the other way around movement just get slower (which is what you want for slow motion).