I wouldn't take anything D&D wrote after season 4 as confirmation of what is or is not an expendable plot point regarding the books. They wanted out, were adamant for years that they only wanted to tell seven seasons worth of story, and weren't going to bother setting up new characters and plots when they could just combine and remix. They cut shyt because they didn't want to bother expanding the world anymore or deal with more complications to a growing production schedule/budget.
Beyond that, Griff being the real Aegon or not is irrelevant to the role he is going to play in the books, which is as a rival to Dany, with a better claim to the throne, a Targaryen prince returned storyline to gain the love of the people, major houses backing him, and the support of Varys by the time Dany gets to Westeros. Griff missing in the story left a void in the narrative that they tried to fill in with Jon and Cersei on the show, but it didn't work.
They needed Faegon to already have major support and castles secured by the time Dany gets there to tell that "but the people of Westeros don't love me" story properly. Broke ass, no ally-ass Cersei not being overthrown by a city that hates her and the entire "I don't want it, she's mah queen" Jon/Dany rift seems contrived because those elements are being forced to accommodate a subplot clearly made for another character.