There's clearly a gray area. Knowledge does get transferred when people move from company to company. If someone developed experience in, say, running retail marketing campaigns for millennials at company X (to remove this discussion from the technical domain) and they take a job at competing company Y, OF COURSE they're bringing over both know how and negative know how developed at their prior employer.

That said, it doesn't mean that it's either ethical or necessarily legal for them to go to company Y and immediately do a core dump of every marketing campaign they ran and what the results were. But there's a fuzzy line between doing that and saying "Oh, millennials really respond well to this sort of language, but be careful not to say this."