I don't think they are equivalent. update() makes sense because it's described as a procedure mutating one operand - among other implications, it fundamentally doesn't commute. On the other hand, set union is commutative, and changing that seems strange (and wrong) to me.
I think it'd be more sensible to add .extend() and + operations to ordered sets if that's a useful thing to do to them. It mirrors the fact that they are (more or less) mutable sequences, and it's more obvious what the order of the result is (and + on sequences is already non-commutative).
In any case, I'm evidently wrong about this being a trivial decision. :)