That’s not completely accurate. The death zone is above 7000 meters and one can survive for quite a long time, certainly more than 24 hours. You are slowly dying however- you’re immune system seems to lag or shutdown partially, the body stops healing itself which hinders recovery from the strenuous climb. But lots of mountains are above that altitude and many people have bivouacked in those conditions.
Also people react differently at high altitudes, and researching the effects are difficult. Even the Sherpas with their survival genetics occasionally get sick when they travel to low altitudes and return to 4000m, but inconsistently. I highly recommend visiting the high altitude research clinic in Periche, Nepal. Their short seminars are fascinating. It’s a small detour on the trek to Everest base camp, maybe two weeks of hiking to get there. Well worth it.