Intuitively, a calculator is not superhuman.
A person who could do mental arithmetic as well as a calculator? That would seem to be superhuman.
If you find out that they were using a calculator under the table the whole time, they're not superhuman any more.
So I think the word "superhuman" must imply a fair competition, in the sense that the participants are competing using comparable approaches. For some definition of comparable.
To be honest- I don't have one. My intuition is that we can't have a good definition of "superhuman intelligence" because having one would require us to demonstrate superhuman intelligence ourselves. Which is obviously a contradiction.