What that engineer is saying is absolutely true for a new engine, not an old engine. He doesn't go into that, so I'll address it the best I can. So how does an engine change over time?
" So, if you put in a lower octane fuel, and then go drive your car hard, press the accelerator quickly, it is possible to experience heavy knock, and possible engine damage." Here he admits that knock can produce engine damage. This is how engines get there.
Lets say that a new engine from the factory is set with a compression ratio of 8.5. You usually won't hear any knock from a new engine with 87 octane fuel. But as engines age, they experience what engineers call "octane creep". This happens because the combustion chamber, valves, and the top of the pistons, get covered with combustion deposits that get thicker over time. When this happens, because the piston pulls in the same amount of air(roughly) it raises the compression ratio because the reduced area results in a higher pressure than when the engine was new. It typically starts at about 30-40 thousand miles, just like the op is complaining at 33,000 miles. So you start off with a new engine that runs fine on 87 octane, and a few years down the road you scratch your head and wonder why it's knocking. That engineer said that engine timing is set. The purpose of a knock sensor is to back the timing(and hurt gas mileage and power) to stop knocking(at least it use to be). So you start with a compression ratio of 8.5, but with the deposit build up it now 9.0. There are only two ways to address it; reduce the load, or go to higher octane. Myself, I don't want my engine knocking. A gasoline engine that knocks is trying to be a diesel, and it's not built for that. When fuel ignites in a combustion engine it burns, but when it knocks, it explodes. There is a huge difference in temperature and pressure when this happens. The real culprit is the poor quality of today"s fuel. Sometimes even 93 octane will not stop it. Myself, I give the engine what it asks for. This is especially important with the greater load our rigs impose on the engine. Myself, when my engine "talks" to me, I listen.