Well, thats a pretty general claim I would not sign up to. In some areas Mantas can be shy of light, so if the dive guides gives that alert, I certainly turn them off - just happened to me last week at Manta Point at Addu-Atoll. Most sharks dont mind. Many turtles and certainly batfish are even attracted by lights and like to closely inspect them.
I tried video lights on hammerheads and blacktip sharks they were scared like hell and so I gave up using video lights. Frankly I don't like having a buddy with really strong wide angle video lights, strobes are fine as they are not continuous.