my understanding is that any Debian/RPM-based Linux running sshd would become vulnerable in a year or two. The best equivalent of this exploit is the One Ring.

So the really strange thing is why they put so little effort into making this undetectable. All they needed was to make it use less time to check each login attempt.

In the other hand it was very hard to detect. The slow login time was the only thing that gave it away. It more seems like they were so close to being highly successful. In retrospect improving the performance would have been the smart play. But that is one part that went wrong compared to very many that went right.

You don't need a "ec2" user. A backdoor can just allow root login even when that is disabled for people not using the backdoor.

It just requires the SSH port to be reachable unless there is also a callout function (which is risky as people might see the traffic). And with Debian and Fedora covered and the change eventually making its way into Ubuntu and RHEL pretty much everything would have this backdoor.