a printed copper thrust chamber successfully completed a series of hotfire tests... That info leads to more questions. I believe copper melts well below rocket exhaust temperatures. The thrust chamber, whatever they are made of now, gets red hot. Perhaps the copper is not the exposed-to-exhaust layer.
Therefore, temperatures used in rockets are very often far higher than the melting point of the nozzle and combustion chamber materials (~1,200 K for copper). Two exceptions are graphite and tungsten, although both are subject to oxidation if not protected. Indeed, many construction materials can make perfectly acceptable propellants in their own right. It is important that these materials be prevented from combusting, melting or vaporising to the point of failure.
This is sometimes somewhat facetiously termed an "engine-rich exhaust". Materials technology could potentially place an upper limit on the exhaust temperature of chemical rockets.
Many fun details here:
https://en.wikipedia.org/wiki/Rocket_engine