Magnus specifically mentioned Firouzja as an opponent that he would consider playing the WCC against[1]. So while not the most important factor in his decision, the fact that Nepo won the candidates again and was the last challenger before Magnus made the Firouzja statement seems relevant. Also relevant is that the candidates tournament concluded on July 5th and Magnus released his withdrawal now in the same month.
It seems to me like the challenger being Nepo was the last significant piece of information towards his decision. How specific it is to Nepo or a more general feeling of Nepo being one of many people he doesnt feel any personal benefit from playing against, who knows.
[1] https://chess24.com/en/read/news/magnus-carlsen-unlikely-i-w...