It looks to me like the root cause was reusing an SSH key on a secure system and a public system. If the SSH key was compromised, that would lead me to believe

It looks to me like the root cause was reusing an SSH key on a secure system and a public system. If the SSH key was compromised, that would lead me to believe that the private key was on the public/insecure system. That is a big security no-no as well.