Maybe a good way to think about it is as a breed. The feathered dinosaurs can't be chickens because A) They don't meet the breed description and B) They don't breed true. BUT they still layed the egg that hatched into a chicken. Much like a black marans and and a cream leg bar are not olive eggers, but their offspring IS because we've got that definition.Well, if the feathered dinosaurs layed the egg, which became a chicken, then wouldn't the answer count as chicken because the ancient featured dinosaur chicken layed the egg!
Now whether or not the egg itself is a CHICKEN EGG is a bit trickier - I'd say it isn't a chicken egg but it still hatched out a chicken, much like it's not an olive egger egg, but still hatched out an olive egger.
But either way, the egg was there and the chicken wasn't. The egg existed, even though the olive egger didn't yet exist.