Hello Juan
The rule is that RFC1918 IP addresses are not reachable or routable on the internet. That is a rule that ISPs are responsible for adhering to and implementing. There is no technological inability to routing these addresses, but by definition, everyone is required to adhere to it and that is why you cannot do it. But even if an ISP does accept them, when they try to hand them off to other networks, the routers on the Internet at large are configured to drop any such traffic.
Cisco’s OCGs mention this because it is expected that all Internet networks are configured to adhere to this rule.
I hope this has been helpful!
Laz