That's probably true. I have a 13" vlamingi, 11" male and 9" female naso and a 9" sailfin. I've only had them for a year and the guy I got them from had them for over 3 years in a FOWLR setup.
Because of the behavior I've seen in these fish as well as some 5+ year old fish I've had in my other tanks, I beleive that they are/have been "conditioned" to only eat certin things. In other words they have come to know that I am the one they get food from and I only feed certin types of foods.
What brings me to beleive this is the large tank with the large tangs that I aquired. The tank was a FOWLR, no corals at all and little to no alage on the rocks. The fish were fed for years nothing more than sinking pellets and sheets of nori.
When I got the setup and had everything settled in, I tried to feed the fish frozen cubes of food that I had used for years with my other tanks. The fish would not even take a second look at teh food floating on teh surface. As soon as I put in some sinking pellets or defrosted the frozen food so that it would sink, they would eagrly eat it up. If I placed a sheet of nori in the tank which floated on the surface of the water, they would eargly eat it even though it was floating on the surface. This is what has me to beleive that fish in our system are "condition" to eat or behave in certin manners.
In my other tanks, I've always just dropped in a frozen cube and the fish would eargly eat it as it floated on the surface. Another thing of note is that since I would feed my fish, usually 3-4 times a week, they never ate or even sampled any of my corals. Again these are just my observations and experiences.