Sushi in the us has to be held at -20C for 7 days, or -35C for 15 hours. They absolutely sell extra cold freezers that go to that temperature. Your home freezer won't.

> I’d wager the vast majority of sushi restaurants are using regular freezers.

After the 7 days at -20 the fish can just be transferred to a regular freezer. So you're right, most places probably don't have the special freezer in house.