So, the point is that margins on the UI boundaries of a component are a bad idea. Margins are a visual side-effect, which break the idea of keeping components encapsulated.

It's not anything new really, everybody has experienced pain to try to reuse a component but it's badly encapsulated so it's painful to fit in a new context. It's always worth making it clear and reiterating basic principle under different lights, but this "considered harmful" like you discovered a new law of physics always feels pretentious to me