I've had my clown for 4 years. Moved him from a 45 g to a 125g 2 or 3 years ago. In those 4 years, he occupied one spot except during feeding time. I haven't seen or heard of a clown who spends all day darting around the tank unless they are being chased or threatened.
I also watch my tang. He is never in one spot for too long. Always moving and grazing. It's in his nature. It's not a matter of guessing a fish's needs. What I know is that these requirements are accurate, given what I have personally observed. plus you really don't know how long these fish last in a small tank. for all you know these people who shove their tangs in small tanks consider a year of keeping them a success. that is not a success considering a tang could possibly live for 10 or more given the right conditions.
A cliche, but accurate saying among reefers is sure you can put a tang in a small tank and it can survive. but it wouldn't thrive. it's like a human being in a closet all their life. give them food, water, and a toilet, they can survive, but it's not much of a life because by nature, we need interaction and freedom.