I think they can be explained although I generally agree that there is not a cut and dried method to decree this machine as flow and that machine is stop and go.

I think flow games typically have ramps that at easy to hit and return the ball via a wire form back to a flipper. But rules are important to setting the tone. Flow games tend to have objectives that take advantage of the flow-y nature of the layout. Example, chaining shots into combos or hurry ups.

Stop and go layouts do not bring the ball back to the flipper as easily and the shots may be more difficult, requiring you to get the ball under control, stop and make a planned shot. Again, rules contribute to a game’s reputation as a stop and go title.