You're correct. It doesn't have to be the constituency in which they resided before they left Canada.
The legislation provides alternative options such as a spouse's address, or a relative's, or a dependent's. There are these types of choices. I think there could be a debate as to whether it's proper to maintain that type of selection or those options for those electors. I would point out, however, that once they make that selection, it's frozen. They cannot change it. They could not be shopping, if we can use that expression, at the next GE. . Once they've opted for an address, their last address in Canada, that's the one that remains for the purpose of voting until they resume residency in Canada.