Releasing a SDK for a game does not mean to gain access to the source code itself. In the today's computer gaming term a SDK means the possibility to create new additional content in high quality for a game. One should not succumb to the erring faith to be able to fix bugs with a SDK for SHIV.
In contrary to other games I see however a problem with Ubisoft. A SHIII/IV SDK for the Community must be created and provided first because it was not planned to release such a complex thing with the game. The Ubisoft inhouse tools used by the devs for the development of SHIII/IV are also used for some other Ubisoft projects and such things are not released to the public.