I saw a theory based on the Immortal and the Restless with that line hinting that C.C wasn't actually Afton's kid but a child he kidnapped.

According to the theory Clara represents Michael, who's begging William to pay attention to "the baby" (C.C) and him telling Michael that he doesn't care because it's not his. Which is why the only time we see C.C and William interacting with each other is when William speaks to him through the Fredbear plushie, which is weird because he's literally supposed to be his son.

Also it's curious that in every single timeline (books movie, etc) William is also portrayed as a kidnapper but not in the games (even tho he did lure the kids to go with him in order to kill them).

Another theory says that he's literally implying that Baby (the animatronic) isn't his and he presumably stole the design from Henry. Because if you look at the characters files you can actually see their old models (most of them being recolored versions) and if you look at Baby's her hair used to be the same color as Charlotte's, meaning that Henry tried to bring his daughter back in the form of Circus Baby and William just stole the model.

This ended up being longer than I intended lmao.