It's not just a US concept. Norway seems to have fairly strong protections on the expression of ideas - i.e. you can't copyright ideas in many cases. I wonder do they have special laws for trade secrets. Opera are going to have difficulty hiring people if they are suing former employees.
This is a trade secret case. If he genuinely had trade secrets (the bar for that is pretty high in the US), things like work for hire, non-competes (which are officially allowed in the US outside of California to prevent the "first bite of the apple" of trade secret disclosure), etc. are irrelevant.
It seems it's a Norwegian case - they seem to have pretty strict laws for trade secrets laid out in the above pdf. The claim is given in Norwegian Krone.