The idea that the prequel pilot is filming on the heels of Game of Thrones wrapping on filming would seem to suggest otherwise, and that viewers may see the prequel not too long beyond a year after GoT ends.
The news that the prequel project is going to film in the fall is surprising, as previous reports made it seem as though HBO might bide its time waiting to get another project ready.
Author George R.R. Martin did briefly mention filming was set to begin on the project not long after its pilot order, although with HBO stating it could be a year beyond Game of Thrones officially ending before a new prequel series happened, it was difficult to know what to believe.