This is not true by fiat or by experience. In ten years working at a company with 10-20 coworkers, we rarely talked salary; only in the very end of my time there and during covid were people coming out a little bit more about it and only in tight knit groups.

What makes you think people always discuss salary? Because it seems rational from a negotiating standpoint?

Of course in tight knit groups. But one tight knit group shares with another and before you know it, it has spread through the network. Happens more often than you think, even more so with younger people.