That article makes it clear that the NAT Gateway needs IPv4
> The NAT gateway recognizes the IPv6 address prefix, extracts the IPv4 address from it, and initiates an IPv4 connection to the destination. As usual, the source IPv4 address is the IPv4 address of the NAT gateway itself.
If the NAT Gateway doesn't have an IPv4 address it won't work.
> The NAT gateway recognizes the IPv6 address prefix, extracts the IPv4 address from it, and initiates an IPv4 connection to the destination. As usual, the source IPv4 address is the IPv4 address of the NAT gateway itself.
If the NAT Gateway doesn't have an IPv4 address it won't work.