I think the poster assumed that everyone would see it was obviously not meant literally.
Just to check, when you said it was a dog whistle, did you literally mean that it was a noise only dogs could hear?
The meaning is pretty clear to me. "If people weren't so ignorant of other games, D&D would never be their favorite game."
Because otherwise people would just say what they mean. "Some people play D&D even though it isn't their favorite game."