Because interrogators SAY stuff like that, hoping to break the guilty party into a confession. Their hope is that he feels so bad about it, that he will just confess to doing something "accidental", confirming that he in fact killed her. They don't care if he says it was an accident when it wasn't - they just want him to admit that he did SOMETHING to the baby. And, many guilty people will respond to this. They want to get it off their chest, but they don't want to admit they murdered, so they will agree it was an accident.
He would not be thinking about the fact that someone saw the baby later, and the cops wouldn't care. They would use that later to get the TRUTH.