100% quarantine is best practice, however it is not the only means of being successful. Ich management can be successful, but is more difficult with Acanthurus than other fishes.
That being said, I have a client that has a midas touch with them. He never quarantines, doesn't run uv sterilizers, and is pretty lax on water changes, however he always has the nicest powder blues and achilles tangs. However, they are in his office and he is able to feed them frequent (10+) small meals a day.
Another customer I have has a number of Acanthurus tangs and also does not quarantine and they have gone 3+yrs without ich or velvet.
Diatom filters and uv sterilizers definitely eradicate any parasites that pass through, and often reducing the number of parasites that way is enough to keep healthy fish from having an outbreak.
Think of humans, we are surrounded by pathogens, but most of the time are able to avoid disease outbreaks ourselves, not by complete quarantine, but by decent health practices. Some have better immune systems than others, but nearly all of us can avoid living life in a bubble.
Some of us in our fish care have failed many times along the way of finding what practices work for each of us. To say they one way is the only way, even if that is the only way you have succeeded despite other attempts is dogmatic and, perhaps true in your case, is frankly not correct and unfounded by science.
But if you wake up one morning, and fish with tiny white dots all over their body are dying left & right - at least have the sense to realize it's time to start doing things differently. 
