Do you have a link for that? Couldn't find it.The precaution from Panasonic is just the lawyers covering their ass.
Sea and Sea does not seem to have that limitation: Note below from the linked chart.
[*1 Number of flashes and recycle time depends on the battery brand, temperature and frequency of use. *2 Number of flashes and recycle time of Ni-MH batteries were tested on 2400 mAh batteries.]
If they were using 2400mAh NiMh then they were likely using Pro or something similar.
No NiMh should 'off gas' unless it is in some kind of failure mode. UK used to have that precaution for their UW lights. I used them for damn near a decade with NiMh without any problems. Somehow the industry got over their paranoia after awhile.
That goes for Li-on also. Both chemistries do have safety vents, at least the good ones do, but they don't blow until a certain defined pressure is reached. Your device is in trouble if things get this far, a flood being one of the more likely scenarios.
FWIW I have personally experimented with putting NiMh in lights with the seal removed, put in an evacuated zip-lock bag, submerged in water and run until dead. I could never detect
any evidence of H2 gas being produced. Doesn't mean it can't happen, but I tend to think this is more of a paranoia precaution than a real threat.
The only scenario I can see where the Pro would cause problems
is that extra capacity. The D2 is known to occasionally cook the strobe under overheat circumstances. So....EVERY TIME....the strobe charges-discharges-charges > heat is produced by both the strobe itself and the battery. Over time this could be cumulative depending on the frequency of cycling and ambient temp, ie the ability to shed the heat. If you can cycle MORE, and not shed the heat in time you are getting closer and closer to a problem. Since you can get a LOT of flashes out of a set of fresh batteries, if you are pushing your strobe to that level on a single dive you are indeed creating some heat, no matter what the battery.