That's actually not quite right. High octane fuel actually burns slower and cooler which allows the tuner to lean out the mixture and add timing and boost without inducing knock. You will not damage anything by running a higher octane fuel on a tune that was done on lower octane. While you may not get the full potential of running high octane, in situations like drag racing where the car is staged between runs, high octane is generally recommended even with a pump gas tune. The simple reasoning here is that your TMIC will get extremely heatsoaked before the run and on pump gas the engine will almost certainly knock during the run and pull timing. If you run higher octane you add a level of protection and allow the ECU to run its full ignition advance making for consistant, strong runs. I would suggest running a mixture of half 91 pump and half 100 octane in any sort of track situation even if the ecu was tuned on pure pump gas. I also suggest resetting the ECU after filling up with the higher octane gas and doing a couple hard pulls right after the reset to allow the ECU to more quickly learn and push the maximum advance it was programmed for.

Thanks