Source? Several raptors have been fired for long duration and even multiple times. If the chambers would melt that should not be possible.
Of course RUDs of engines happened but they were the exception and not the norm.
Also, the only non-intentional engine RUD that I know of is SN8, where debris from ground destroyed the engine avionics at takeoff. Any other incident?
Elon himself said this was one of the major remaining problems in last night’s presentation.
But I think what he means is that currently the chamber gets to much damage to be full reusable on a daily bases over a long period of time, not that it doesn't work at all.