Skyrim has not the same engine as Oblivion and Starfield has not the same engine as Skyrim. There always were huge upgrades and changes to the engine, saying that Starfield has the same engine is like saying that Unreal 5 is the same old engine as Unreal 1. It is the same engine in the same way as I am the same as my father or grandfather. We share lots of features and DNA and have the same last name, but we are very different in many ways.