"To leverage the psychology of your brain, you need to instead choose clear goals that you clearly know how to accomplish, and then approach scheduling with flexibility."

What if the goals are unclear or you don't know how to accomplish them? If the clear plan is to make goals clear, then you might as well put that on the schedule and work on it for the allotted amount of time. Hence "write every day".

Chekhov tied himself to the chair as an act of self-discipline and wrote at least one page a day. This seems to have worked well for him.