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An engine can still knock with a knock sensor but the knock sensor prevents it from knocking more than that until conditions change. With a knock sensor the octane doesn't affect engine life as much as it affects power. Current Generation Premium fuel engines will run on less than Premium but their power output is less. Without a knock sensor running too low of an octane fuel will kill an engine before a high octane motor.
The only thing I can believe he is referring to is a little understood event when a high octane fuel(like alcohol) is used in a low compression engine and the timing is retarded past a certain point. The idea is that the fuel may ignite into the exhaust stroke before it completes combustion and damage may occur. In this case I'm speculating but some of this may have to do with lean fuel-air mixtures which reduce flame speed. Engines that try to run alcohol may retard under, I forget what it's called when this happens, but it actually makes it worse. I actually believe that an alcohol motor needs to have more timing advance when it's run lean.
And with that I doubt he has any idea of what he is talking about. Recommended octane is a minimum value. Going a few octane higher will not lead to shorter engine life. Running unleaded Race gasoline in a motor isn't going to kill the motor from high octane. Unleaded Race Fuel only goes to 99-103 octane anyway.
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-Allch Chcar
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