The `100.64.0.0/10` address block is **not** *private* address space; it is *shared* address space. This is spelled out in [RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space][1] (I highlighted the relevant verbiage):


> 1.  Introduction
> 
>    IPv4 address space is nearly exhausted.  However, ISPs must
> continue to support IPv4 growth until IPv6 is fully deployed.  To
> that end, many ISPs will deploy a Carrier-Grade NAT (CGN) device,
> such as that described in [[RFC6264][2]].  Because CGNs are used on
> networks where public address space is expected, and currently
> available private address space causes operational issues when used
> in this context, ISPs require a new IPv4 /10 address block.  This
> address block will be called the "Shared Address Space" and will be
> used to number the interfaces that connect CGN devices to Customer
> Premises Equipment (CPE).
> 
>    Shared Address Space is similar to [[RFC1918][3]] private address space
> in that it is not globally routable address space and can be used
> by multiple pieces of equipment.  ***However, Shared Address Space has
> limitations in its use that the current [[RFC1918][3]] private address
> space does not have.  In particular, Shared Address Space can only
> be used in Service Provider networks or on routing equipment that
> is able to do address translation across router interfaces when the
> addresses are identical on two different interfaces.***
> 
>    This document requests the allocation of an IPv4 /10 address block
> to be used as Shared Address Space.  In conversations with many
> ISPs, a /10 is the smallest block that will allow them to deploy
> CGNs on a regional basis without requiring nested CGNs.  For
> instance, as described in [[ISP-SHARED-ADDR][4]], a /10 is sufficient to
> service Points of Presence in the Tokyo area.
> 
>    This document details the allocation of an additional special-use
> IPv4 address block and updates [[RFC5735][5]].


  [1]: https://tools.ietf.org/html/rfc6598
  [2]: https://tools.ietf.org/html/rfc6264
  [3]: https://tools.ietf.org/html/rfc1918
  [4]: https://tools.ietf.org/html/rfc6598#ref-ISP-SHARED-ADDR
  [5]: https://tools.ietf.org/html/rfc5735