That rule of thumb usually refers to having unfettered access to the hardware - to be able to crack it open, snoop on internal signals etc.

In this case the problem is that the dock is expected to be a safe interface (untrusted), when it actually isn't. For ex, people would be surprised if their computer could be hacked by plugging it into a malicious power socket. And likewise they'll be surprised if they find out their phone can be hacked by putting it on an alarm-clock ipod dock in their hotel room.