> My argument is that they are not conceptually different.

Your argument is nonsensical on its face, it could not be a more different way to compose types.

> The implementation is different

It's also largely irrelevant, the commenter above makes no mention of the memory state, because that's not the point. The point is that the states are exclusive.

> The compiler can force you to handle multiple return values the same way it can force you to check a variant.

No it can not, specifically because it's a product type (or it would be if it were a type anyway), and there is no relation between the fields. At most you can apply some mangy heuristics and hope they don't fuck up to often.

A sum type tells you in no uncertain terms that you can have only one of the types at any given time.

And that's not accounting for the fact that a result is a reified value, which an MRV is not, so it can be passed around and manipulated as any value, and can be operated on as a thing of its own.