It correctly dismisses the idea that lack of oxygen in the air inside the bag would cause problem with a low stay time of 10 minutes.
He's using a low value for maximum CO2 concentration. 5% is toxic, but you can withstand it longer than 10 minutes. 3% is bad, but headache-inducing bad, not suddent death bad. As you mention, it starts to get bad. He has a figure of 375 mL/min, which is higher than my earlier search found (200, but at rest, so maybe he expects people to exercise a little in the Bag).
1.8 m^3 x 3% = 1800 litres x 0.03 = 54 litres of CO2 in the bag.
With an expel rate of 375 mL/min, that's 54000 mL/375 mL/min = 144 minutes. There is no risk of sudden death after 10 minutes due to CO2 at sea level pressure. Especially since the 3% threshold would be reached after more than two hours, and not be immediately lethal at this point.