I'm aware that this particular code was - but presumably whichever hypothetical person has arbitrary binaries pushed to them in the grandparent's scenario was using a Mac or iOS.

Server A is trusted by server B. Danny uses his iPhone to manage server A - ie he accesses server A in a privileged state from his iPhone. If we treat his iPhone as compromised, then the rest of the chain of trust should be treated as comprised all the way through server B.

And that's the more difficult exploit to engineer. If Danny is using OSX for administration and we treat his installation as compromised we get a much richer stack for exploiting server A.