According to interviews he did give them the broad strokes of how it should end, D+D are just so incompetent that he needed to give them a lot more for them to follow.

They also literally had a grudge against Emilia Clarke over her renegotiating her contract so she wouldn't have to do nude scenes anymore back in the second season, which is when they started writing the end of her character's story as a revenge fantasy against her personally. So it wasn't even just incompetent writing, but them actively deciding early on to write a weird, misogynistic ending out of spite for an actress not wanting to be naked around them on the set.