The dummy battery connector he’s using apparently has an active circuit inside and is designed for 7.4V-8.4V input, in which case 9V output from the battery is probably ok. Of course, that’s based on sketchy Amazon specs which also say “4V-8.4V” without specifying for what. I don’t put too much faith in those specs….
The real voltages are probably higher unless under heavy load, and there are probably regulators inside the camera that make that ok anyway. Those regulators might run hot, but having the battery outside the camera removes a major heat source.
In sum, probably ok, but I don’t think I’d risk it with a $1000 camera unless I was truly desperate… even then I’d test it before trusting it.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.