I don't think that's true. It doesn't take magic to do just about anything over USB. It's just a rather stupid serial bus protocol, after all.
Now if you assure me that your "simple external device designed specifically for security" is impenetrable and incorruptible when it is attached directly to a processor bus inside the computer, then I'll believe you when you say it remains so when attached via USB.
But I have personally watched as corrupted USB devices attacked a computer in seconds, and as a fresh, uncorrupted USB device in another USB port was turned into a similar attack weapon. I know that's not exactly what you're talking about, I'm just sayin' you can't take anything for granted, including a USB attachment.
Yep, anything that connects directly to the system bus - usb, firewire, thunderbolt - can theoretically get to the computer’s main memory and it’s game on from there.