But what if you sold it for cash? For all Apple knows that's what happened in the poster's case (he sold it, then somehow got physical access again, then wants to steal the other person's data)

I really see no way for Apple to support the use-case of someone who a) doesn't want to use Find My and b) loses their Mac where another party activates Find My and c) regains access.

You really need to be in the a+b+c situation which is quite rare.