If we want to send ping from an EC2 instance there must be outbound rule (in security group attached to that instance) that allows echo requests. Since the security groups are stateful response traffic for that request will be allowed regardless of inbound rules. There’s no need for a specific inbound rule that will allow echo response. EC2 that is receiving this ping must have security group attached with an inbound rule allowing echo requests. Analogically, there’s no need for a specific outbound rule that will allow echo response.
So why is there an option to create rules for echo reply?
It looks like it doesn’t have any use. The only thing that comes to my mind is to allow sending echo reply without receiving echo request, but this doesn’t seem practical or even correct use of ICMP.
There is no obvious reason why this option is permitted, other than perhaps for completeness, or because of unusual cases (admittedly, none come to mind) where an unsolicited echo reply might be expected or desirable.
As you indicated, the stateful nature of security group rules should make actually using this feature unnecessary.