EasyJet Switzerland operate flights under a franchise arrangement with EasyJet UK. Those flights share EasyJet's U2 code.
However EasyJet Switzerland do have their own code DS allocated by IATA (see
http://www.iata.org/publications/Pages/code-search.aspx) which they would use if they operated any flights outside of the franchise arrangement (though I don't think they do).
So everyone is right, sort of.