They spend most of their time at sea at snorkel depth. Sub suck on the surface as they wallow like a cow just before giving birth to twins.
snorkel depth allows them to operate submerged and still get the air they need for the diesels.
If as noted in the article they had battery issues then this could be catastrophic - as battery failures can lead to toxic fume release.
IF they had 20 ft seas at the time and the snorkel couldn’t get enough uncontaminated air, I could very easily see them foundering while attempting to address the battery casualty.
Not correct. Diesel subs cannot run all of their engines while snorkeling. They do not have enough battery power to stay transit submerged for long distances and make long transits on the surface.