I think what is throwing some people off about the Falcon's turret rotation is that when you think it through, in the ANH turret scene the guns are pointing straight away from Luke & Han which means they're pointing straight up and down relative to the Falcon. So, going by that, being able to point straight back shouldn't be possible, at least not when manned as that would mean the guns would then be pointing straight up and down relative to the gunner. But as others have said, who knows what upgrades Han has made to the Falcon since the end of Jedi and it's even possible that the turrets are now operated remotely.