Based on all that I've read, the above does not suprise me at all.
In any normal ops, one gets nowhere near the CHTs required for detonation.
I can however imagine there are people out there who do manage it, due to stupid operating practices (like climbing at Vx all the way to cruise level) combined with having no engine instruments.
The highest CHT I have ever seen on mine was 430F and that was departing from Turkey at +35C, after a long wait on the ground with the engine running, and having to climb fairly steeply as well. And the TB20 does suffer from less than great engine cooling. Normally one doesn't exceed 400F; ~360F in cruise.
The strange thing is that TOTAL seem to be charging almost the same for 91UL as for 100LL. I expect they are hoping to push down 100LL volumes and once enough airfields stop stocking it, they can then screw the market.