Against oxidation, a soldering flux like rosin is very effective. Upon heating, it de-oxidises copper alloys, permitting the solder metal to wet them. Maybe the producer uses a similar compound (but without leftovers) for metal clay? This would permit long storage, needing a reductive atmosphere only in the kiln.
Sintering: it can achieve the full strength of an alloy, but only if using extreme pressure, hence my interrogation. Some alloys are produced that way, especially refractory ones which would be impractical to melt: molybdenum, tantalum, tungsten...
Produce a powder: wearing now my mechanical engineer hat, I'd say "grind it". Though, many metals catch fire then: titanium, iron. Neither did I try with gold, which may well clog the grinding wheel and produce zero powder.
I wouldn't be surprised if only a mix of powders with different grain sizes sinters well without pressure, because such a mix leaves less void between the grains - just like concrete uses sand, gravel and crushed stones.
Thanks for the links and explanation! I didn't know such materials. Maybe I find uses in industrial applications.