UV lowering the disease risk is a broad spectrum term for BS. Most common aquarium rated UV systems don't produce the UV length or intensity to control even common diseases, and a few of the cheaper ones will barely control algae.
It may 'help' but to what extent is debatable.I was told it would help with white spot ...
That is probably a discussion you should have with the LFS :rolleyes... why do they say that if it isn't right