I think any fish requiring such freezing likely would have been frozen to spec before it got to Costco or any restaurants. I’ve never asked any of the seafood purveyors I know about it, but I do know you can buy the stuff from them and use it immediately. I don’t trust the government for much but when it comes to being overly regulatory on food safety they’re pretty tough to beat. If it’s being sold ready to use I’m sure it was processed before, and there’s a HACCP plan in place to ensure it.