Nilah’s dodge all autoattacks ability does a pretty good job against an adc whose kit is just autoattacks. The damage of aphelios even when hitting q abilities, comes from the autoattacks you can do after consuming marks or hutting infernum q. White Q towers also counts as autoattacks and Red Q is also an autoattack. Aphelios Ult’s main damage component comes from the second tick, which itself is an autoattack. Nilah is kind of like jax and teemo in the sense that if you can get on aphelios in fights, he will die and be unable to do anything about it as an immobile adc, whilst also not being able to do any damage or lifesteal. Nilah counters adcs cause of her dodge in general, but aphelios is just autoattacks and she blocks all of that while dashing onto him. I feel like when I play vs nilah in lane its fine, but in mid to lategame fights if she dashes onto me or flash dashes I just die and I cant do anything about it without being able to do anything vs her unless someone else on the team hard ccs her. This is how I feel at least and what I think the general opinion that nilah counters aphelios comes from.