Earlier quoted context omitted.
> they know from a long way away what watch brand is on that guy's wrist No, they didn’t. The makers of movements and makers of cases were separate. From far away you only know the case on the wrist. Not the movement. (I think Rolex was the first mass-market Swiss watch brand to vertically integrate. Patek may have been the first boutique.)
The movement isn't part of the brand. It's not part of the signal. The case/dial/sometimes band are the brand. And if you couldn't tell them apart, they wouldn't be any good at signaling, the entire point of wearing them.
The watch manufacturer, as part of their reputation, buys “premium” internal components. And then the hardcore watch-heads get to know that this model has that premium movement. Everybody in the club gets to signal to each other by knowing internal details that outsiders don’t notice (or even details that can’t be noticed, I mean, I assume by nowadays non-premium-brand movements are functionally identical to the premium ones).