With internal temps too high, you can get pre-ignition this certainly can lead to detonation.
Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?
The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!