That may be true ( quality / technology of newer belts vs ones from decades ago ) but new or not, if not re-tensioned after that initial post-installation stretch, I hear they can get too loose and move just enough to risk the edge of it get sheared by the pulley...and then the result can take out your timing belt . Likely a rare risk, but still a risk ..
My ex-mechanic always said that's b.s. because there's no space in there whatsoever which would allow the belt to move even a mm away from the center of the pulley, so he was only concerned that if the belt is loose it could skip a tooth and you'd end up with a vibration . Either way, a belt check is a good thing to do .