That's a real method:
https://digital-photography-school.com/taking-photos-in-busy...
https://petapixel.com/2019/09/18/how-to-shoot-people-free-ph...
But it only works well if the crowds move out of the way reasonably quickly. If we're taking about areas packed with people all blocking a certain area, and you need hours of shots, the change in ambient lighting over time will have negative effects on the end photo.
Ah, that's the method indeed! Thanks!
There's an analogue method too - just do a very long exposure of a busy street. There is so little reflected light from people (relatively speaking) that it barely registers on the film/sensor, hence they are 'invisible' in the final shot. That's why old street photos are sometimes empty of people, because both lenses and film were slow, leading to long exposures.