He’s right - but if you feed pilots air at a cabin altitude of 8000 feet (much lower in oxygen content than air at a real altitude of 8000 feet) then when the chips are down crews will make mistakes - and they did.
First, there is no call or need to don the O2 masks in this case. Second, you need to read up on the partial pressure of O2. The O2 present in the blood is the same, whether in a plane at 8,000' or on a mountain at 8,000'.