You seem quite sure I am arguing with you.Yes I have. It is not mandatory to pre-book, you will not be refused entry, just given a form directing you to make a booking. But in either case, nothing would stop you from contacting the lab and booking a normal travel PCR instead, whether pre or post-entry.
I don't get what is up today with these petty objections and gainsaying, you are usually very astute. I never suggested this works in all cases for trips of all durations to all destinations. As I am reiterating for the fourth time, all I said was this could work if it fit your window. The reality is that for a 6 day trip from the US or a one week trip from the Netherlands, Canada or Aruba et al. you can save time, inconvenience, wear and tear on the nasal passage and some cash if you booked the regular travel PCR test instead of a 5 Day and killed two birds with one stone. Nothing prevents this. It might even give peace of mind to those anxious about not receiving a negative, even if it does not fit their travel window exactly.
My responses are for entry to the US; that is hard enough to track without including the rest of the world.
Your theoretical use of a test on Day 5 to go home on Day 6 (or later if not to the US) is intriguing. I'll be interested to know if anyone has been successful doing this.