I'm not sure about fiberglass, a fiberglass pool might be affected by a low calcium level or it might not. Vinyl will not be bothered at all. There might maybe be some effect on metal surfaces, but if so it is very very slight.
The pool store people don't know, they are just repeating something they read somewhere. There is a lot of misinformation out there. Plus they would really like to sell you some calcium even if you don't need it. At least with calcium it is comparatively unlikely to hurt anything. Some of the products pool stores push on people can cause worse problems than the ones they are intended to solve.