No, it mustn't. That is their opinion, in the same way that somebody here have different opinion. Just two opinions. And probably only DJI knows for sure. Or somebody that knows about motor mechanics.
And speaking of DJI I don't remember anything in Mini's manual about that technique to power off the motors. It could be that powering off motors by flipping the drone is just emergency mechanism integrated in case of unwanted flipping during flight (Emergency Propeller Stop), and somebody just figured out that it can be cool way to use it after grabbing drone in hand. But maybe DJI did not intended it for such usage.
I think that is logical that any drastic change of speed in motors is stressful for them. That is the reason why motors usually gain or lose speed in gradual manner.