Lightly grilling fish won't heat the interior meat enough to kill parasites, though.
I think that there are regulations that require fish intended for raw consumption to have been frozen for one to seven days at -20C, which works out OK for factory ship tuna. Tuna caught by fishing boats are lightly processed in factory ships, being gutted and washed, and then flash frozen to be stored while more fish are brought in for processing. By the time the fish reach market, they will have been frozen for the requisite length of time to kill parasites. Also, some fish parasites live in the gut while the fish is alive, but when they sense that the fish has died, they migrate into the muscles (since predators and scavengers will eat the muscle and act as the next host). This migration is inhibited by cold storage, but is best prevented by quickly gutting the fish after being caught. And so fish that are processed at sea will have fewer parasites in the muscles than fish that have been brought to shore and gutted a long time after being caught.
I would feel that giving a child of the isekai world the impression that fish may be safely eaten raw would set them up for infection. If they did not know that Nobu's fish supply had been specially prepared to kill or minimize parasites, they might try to eat local raw fish. Such fish would certainly be loaded with parasites. It's rather like getting them used to drinking unboiled water. The municipal water at Nobu's would have been tested and purified for safety, but locally-supplied water should be boiled before consumption to prevent cholera or other diseases.
The gap between the Empire and modern Japan is enormous.