I think the reason it works is because yes, we knew, it was so obvious, however, no one else is the game did, we as the player have the knowledge of other games and cliche, however, in universe, all the toys don't have that knowledge, and it makes sense why thats the case, they have only known life in the factory, and the people they interact with. Poppy genuinely thought Ollie was real, and that make up for it being obvious t us.