They were dead before the crash? Must have been a depressurization thing.
Planes of that era were not pressurized. The B-29 and a handful of recon aircraft from late-WW2 were the first examples of pressurized military aircraft. The JU-52 was a very crude aircraft. Corrugated skin like you find on your shed roof. A tri-motor design because 2 engines weren’t reliable or produce enough thrust to move the aircraft and it’s cargo. I’d have to check, but I rather doubt that the engines were even supercharged for high-altitude flying (though the Swiss may have re-engined it on account of the Alps).
Not a pressurized plane unless they modified it in some weird way.