Is this "rule" a guideline or a law? Is this something they want to teach or are they going to hand out ticket for opening doors with the wrong hand?

If this is a guideline, I would suggest that bicyclists (and e-scooters et al) also be instructed to leave distance between themselves and parked vehicles. Motorcyclists have for years been taught to avoid getting so near to parked cars for fear of being "doored". Speeding along only inches from parked cars is dangerous to everyone, especially pedestrians who may be stepping out from between cars.