> RFC6598 defines CGNAT IP space as 100.64.0.0/10

That's a relatively recent RFC; there are ISPs which have been using CGNAT for longer than that, and they do use RFC1918 addresses for their CGNAT (for instance, I know of one which used something in the 10.x.x.x range for CGNAT, which works in practice since most domestic networks use something in the 192.168.x.x range).