Dismounting: to get off your bike and walk besides it in any fashion that does not involve straddling it, in any way or form, or carry your bike.
That way the Observer: aka police or civic duty personnel, can easily distinguish the difference between "riding" vs "walking"
It is part of the enforcement of the law itself, so that there is no, gray area to challenge, whether you were riding vs walking the bike.
Straddle walking: as in two feet firmly on ground and is the only means of forward and backward movement, is quite safe, so long as you are mind-full of who is around you, because you are only moving the pace (walking while straddling) of a pedestrian with the width of your shoulders and length of your bike. this is especially true when carrying loads on saddles, baskets or handlebars. Where walking besides the bike would be dangerous due to the bike fighting gravity and trying to keep it upright, when it starts to go over gravity kicks the tires out to the side when you hang onto the handle bars trying to stop the downward pull. (9 years of direct experience as only means of transportation of myself and goods I carry. It is extremely dangerous for self and others when walking "besides a bike" loaded with a 40-80lb load for example.
When you are not seated and using one pedal or kicking off the ground with the other foot for forward movement, while balance on the other pedal is quite dangerous to self and others. If you need to come to a stop right now. Body in motion stays in motion unless acted upon outside force. Meaning balancing on that single pedal and no contact with seat means the bike is NOT going to stop you when you squeeze the brakes sending you flying forward or other dangerous direction.
In fact its more dangerous than having both feet on the pedals where you lift off the seat. At least them your center gravity is in the middle due to equal pressure on both pedals and there is more friction between you and your bike when you squeeze the brakes suddenly.
Again: Dismounting, is entirely about making is easier for observer to know which you are doing, thus making it easy to enforce the law/ordinance.