Yeah, something doesn't add up. They clearly aren't committing suicide, so the oxygen is clearly not going to be allowed to run out.
Maybe they mean they'll stop when the oxygen gets "rather low", which is around the point you'd stop being able to perform strenuous physical activity because you'd be weak and light headed.
Either way, if the box actually -is- soundproof, why even perform? It's not like anyone can tell the difference between you just sitting in there relaxing if it is.
But wait - how do they power the amp(s) and the electric instruments? Better question - how do you soundproof such a small room against the traditionally high decibels of Death Metal? Would being inside such a small room with such loud music hurt the occupants, especially if sound is unable to escape due to soundproofing and instead reverberates around inside?
Has anyone even heard of these guys before? And why is the Gherkin skyscraper (presumably) allowing and/or endorsing the stunt?