I think my favorite lesson was their term "implementation-driven design" that I still see pretty frequently. It's where you build your UX to match your system architecture and is the opposite of "user-driven design". Like building forms that are just one-to-one with the database tables. It's why I cringe every time I see devs on hacker news saying they don't need managers or designers to build products because you'll end up with an implementation-driven design more often than not.