That was dangerous due to the risk of hypercapnia: "increased CO2 levels could lead to unconsciousness which, if happening while in the water, can result in drowning.". Note that our breathing is regulated by the amount of CO2 in our bloodstream, not the amount of oxygen. There is a fantastic explanation of the danger here: https://blog.daneurope.org/en_US/blog/are-full-face-snorkeli... There are safety regulations to limit the volume of air inside a snorkel (the dead space).
"Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, or the smaller the tidal volume of breathing, the more this problem is exacerbated." —https://en.wikipedia.org/wiki/Snorkel_(swimming)
"Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, or the smaller the tidal volume of breathing, the more this problem is exacerbated." —https://en.wikipedia.org/wiki/Snorkel_(swimming)