why not test mapping rules?

[running910]

According to the definition of rfc3489, "A cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port."
But pystun seems to only send one external IP, and then use this external IP Return, and then use another external IP to send to pystun. If pystun receives it, it will be judged as full cone NAT.
I am wondering why not try to use pystun to send to the second external IP to test the mapping rules? Isn't this the basic premise of the Cone NAT?