With internal temps too high, you can get pre-ignition this certainly can lead to detonation.
But does hot air lead to higher "internal temps"? I'd be surprised if it did, as the power is substantially decreased. I'd wager the CHT would go down on application of carb heat.
Anyone got an engine monitor on a carburettor engine?
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?
http://www.avweb.com/news/columns/186015-1.html
http://www.avweb.com/news/columns/186216-1.html
and an Aussie coroner that was rather more persuaded by Braly and Deakin than Lycoming!
...not to mention some personal experience of reading things in POHs that don't seem to make physical sense.
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!
Empirical data and scientific principles, yes.