This warning by Panasonic doesn't appear to be new. Here's a link to a thread that's 8 1/2 years old, discussing Sanyo's then recommendation that Eneloops should not be used in waterproof lights:
There must have been very many Eneloop cells used in underwater strobes since then. It would be interesting to know what are the circumstances of use in strobes that might lead to hydrogen venting. I would suppose that any increase in pressure within a sealed battery compartment might be a greater functional concern than a risk of hydrogen conflagration when the gas is released?