Sure, but isn't it more likely that a key that has to be shared by who knows how many ssh load balancer machines at GitHub and can't be easily rotated because it's pinned by millions of users, isn't it more likely that that private key gets eventually compromised or thought to be at risk at being compromised?

We need to compare the relative risks within the same context, namely within a company like GitHub

So it's not relevant to bring up failures of other CAs