Perhaps I'm reading the post the wrong way, but particularly in the case of junior v. senior, I see the deadline point to be more about what a designer can do,

Perhaps I'm reading the post the wrong way, but particularly in the case of junior v. senior, I see the deadline point to be more about what a designer can do, given their experience and seniority. A junior designer is going to be handed an assignment and told "we need this next Friday." A senior designer will likely be handed an assignment (or work with a client directly) and be asked "when can we expect some initial ideas?" It's less about what a designer of a certain experience should do, but rather what they can do.