Leading on, were the ac "interfered with" which seems to include Xpdr, ACARS, VHF etc. then I am sure ELTs were considered as well.
The ELT unit itself contains the G switch to set it off. Nobody said they're perfect (and do have a pretty good failure to activate rate). Point is if it's underwater, who cares? The only thing that would bring attention to the black boxes are the pingers, which are water activated.
AFAIK the ELT is mounted outside the pressure bulkhead (usually) and someone would have to go pull the ELT battery out (there's one in the ELT container so it keeps transmitting when it doesn't have ships power after activation), as well as disable ships power. Now you're involving maintenance activities or a large gang of people to go around pulling power and batteries from devices. I know of no evidence pointing to the likelihood of such an organized event taking place here.
And as other non-US airlines have ELTs installed, it's probably safe to say it comes from the factory pre-installed, so one would have been here. Probably something the FAA wouldn't allow them to make a customer option during type certification but I know little of that beyond some paper-pushing on some 337s I had to do.