It is not angle of attack that the rule refers to. It is Q-alpha which is the dynamic pressure times the angle of attack. The shuttle had pre-calculated Q-alpha and Q-beta structural limits; apparently Apollo did too, and this rule applied when the instantaneous value of Q-alpha reached or exceeded 100% of the pre-calculated structural limit.
Edit:
This Q-alpha value was actually displayed to the crew on a meter. (On orbit the meter served as the Service Propulsion System chamber pressure meter.) The description of the value shown on the meter is:
the qa display is a pitch and yaw vector summed angle-of-attack/dynamic pressure product (qa). It is expressed in percentage of total pressure for predicted launch vehicle breakup (abort limit equals 100%).
References
Orbiter simulator graphics for high-res closeup
Saturn V Flight Manual SA-503 for description and panel image
End Edit
Additional info:
The shuttle rules were shown as plots of Q-alpha versus Q-beta, indexed by Mach number. The plots had the strange name of "squatcheloids".
Here is a sample Shuttle squatcheloid showing the Q-alpha and Q-beta limits at a given Mach number.
