Not a lawyer.... If any in-house libraries were used this becomes a problem if changes or updates to those libraries were written during development of Fallout Shelter. Any subsequent versions of those libraries would become tainted and verboten from use in other products. Even any fixes to those libraries after initial distribution to correct bugs in the game could reasonably be argued to fall under the contract.

In practice exclusivity restrictions demand a from-scratch product. Developers are cheaper than lawyers.