Doctor Sleep is being marketed as a sequel to Kubrick’s adaption of The Shining because it IS a sequel to Kubrick’s adaption, not necessarily just an adaption of the Doctor Sleep book.
Flanagan was able to convince King that if they were to do Doctor Sleep as a film, it would have to be connected to the story Kubrick told, otherwise it wouldn’t work with people. Apparently King liked his pitch and surprisingly gave him the okay to go ahead with it.