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If either of the inputs tries to rise to 13.5 volts, the diodes going to the +12V rail will be conducting a HUGE amount of current, which should quickly discharge the extra energy on the line causing the high voltage. This type of protection is meant to remove short duration, high voltage transients (high current for a very short period of time) or long duration, low overvoltage conditions (which would draw a much lower current). In either case, the diodes will survive.
You cannot analyze this circuit assuming a constant DC 13.5V input, unless you are willing to say that the diodes burn up and fail (which is what would happen in a real circuit). The diodes would be conducting somewhere in the range of 100's of amps, indefinitely, and that's far from realistic.