Well probably anything related to "tamper proof" security (defending against attacker who has full uncontrolled access to "secure" device) of anything is security by obscurity at best (security of such commercial products is often based on that each device uses unique configuration of countermeasures) and pure snake oil more often.
In this case whole security of this system really relies on people (and their belief that there are some technical security measures that they cannot trivially circumvent) and focusing on technical means too much would actually divert resources to something that is actually not possible to do perfectly. USAF probably correctly assumed that relying on people is better approach. I remember that some (probably DoD-sponsored, in relation to WMD security) study asserted that only meaningful way to protect system from well funded and motivated attackers is to not let such attackers near the system (presumably by means of guarding it).