That couldn't be the (hypothetical) syntax, since (unlike if let) it would have to shadow object permanently in the rest of the scope — wouldn't it? — a practice that Swift disallows elsewhere as confusing.
Also, by computing an optional result, you would then (presumably) carry the optional value forward into following statements. As I preached here, carrying optionals isn't a great idea in Swift.
If you had to do that, ugly or long-winded syntax would signal your intention more clearly, so it might be the better approach after all!
Edit:
To be clear, the "long-winded" version would have this shape:
let addedObject: Object?
if let object {
addedObject = add(object, for: key)
}
else {
addedObject = nil
}