You bring up some good questions.
There is a certain amount of latitude when selecting tank size. In addition, a very experienced reef keep may be able to get away the doing things that would result in a tank of very dead fish for a typical reef keeper.
Also, there is the difference between the fish surviving and it thriving. It's a lot easier to keep a tank in a larger tank, if for no other reason, tangs are big messy fish that produce a lot of waste. A larger tank can more easily deal with it. I wonder how many tangs, kept in 70 gal tanks are really thriving, and living for years. I suspect it's comparatively few. It also seems to me that many of the people using 70 gal or smaller tanks, don't have anything bigger are the same ones that are saying it's ok for a tang.
As for what you see on vendor web pages for requirements, is often a questions of how honest they are verses how bad they want to move the fish. Lower requirement makes for more sales. Also, a lot of times they copy the same information from one fish profile to make the new one.
Lastly, what is expressed in the posts here are opinions of the various posters. There is bound to be disagreement in some areas. You get to figure out who you want to trust, but don't make the mistake of trusting them because they are telling you what you want to hear.