Its only recently been shown as an agent of the Reach, back in the Jamie Reyes days...
In any case, the Scarab has hacked other Reach computers, so they're not hack proof tech. And Reed is one of those who given enough time, could do it.
One of those endless no limits fallacies, I guess. Reed can hack XYZ, so he can hack anything, vs Scarab itself has never been hacked, so it never will be.