Concur,
I DID learn from an old timer (my dad) who was diving up to 90 meters, in air no deco gas, no dry suit, no bcd.
He was relying on a couple of shoppers for extra lift, but he was not doing it with a single 12 liters tank, he was putting together 3 12 liters tank together each with its own regulator.
My first dive (36 years ago, I was 14 and I remember it like it was yesterday) I went down do 14 meters the next day I went down to 34 and then 30-40 meters was my 'normal' depth in air, using a twin set (no manifold no bcd but an spg!)
just for completeness, my father was not crazy I was a free diver diving in the 20-25 meters range for spearfishing and I had been doing that since I was 8, so I was kind of skilled underwater.
Diving deep air is not a problem, but you need training and experience to deal with nitrogen narcosis. My first bout of tech diving was getting certified tech deep air (deco on air deep as 60 meters).
So diving deep on single gas being air is not foreign to me, what I find strange is the lack of risk management.
Cheers