I agree. It would work in a 75g though. the six foot rule is for most, not all, Tangs. Bristletooths are fine in 75s...i beleive most of the mimics that i have seen are as well. they just behave differently and therefore have different requirements.
There seems to be a lot of posts lately with Tangs getting HLLE
and they are always in small tanks. There are almost daily posts of "my Tang has Ich..." and they are almost always in smaller than recommended tanks. Stress causes problems for all fish. Tangs, seem to take it worse.
There are always exceptions. The Hippo you talk about...its a 8-12" long, as it should be at that age and its in an 18" wide tank, so it can swim a couple of inches before it has to turn? Of course, this is owned by someone that pays to have a small tank maintained. Sad.