Time of consciousness at FL 250 is 3-5 minutes, FL 300 is 1-3 minutes, yet people can and have climbed 29,000-ft Mt Everest without supplemental oxygen. But no-supplemental-O2 efforts require a lengthy acclimatization period at altitude.
No depressurization, intercept pilots were able to see the pilot slumped over, windows would have been frosted over if a decompress, most likely a medical issue.