I'm 99% sure that comment is referring to classes with multiple dps specs; not comparing 2 different classes. E.g., keep fire mage best mage spec.
In no way am I saying this heavy of a nerf was justified, but that philosophy being held onto I'm pretty sure is basically only for pures. (Main swapping in the past was ALWAYS a tough job even if it is harder now. Spec swapping in Legion is infinitely harder than any other time previously)