I think you are misunderstanding. His sentiment seems to be one of "I am burnt out and unmotivated to continue BUT the idea of playing against a young upcoming

I think you are misunderstanding. His sentiment seems to be one of "I am burnt out and unmotivated to continue BUT the idea of playing against a young upcoming GM could be motivating enough" So it isnt the most important reason, but nepo being the challenger didnt offer anything to change his mind like he was hoping the result of the candidates would.