He didn't fall 150 feet, nor was he burnt to a crisp.
Nope, he never died.
1. Armin fell about 50 meters from the Colossal titans teeth, via they are about as tall as the wall. He then falls on a 3 story building shown to be about as tall or taller then Eren and reiners titans. So at worst he fell 35 meters.
2. Humans can walk off a fall of up to 17 meters, from up to 30 are certainly deadly, but not instantly. Which is the key here, there's no doubt that these things would kill him, but first he'd been in a coma for a few hours.
3. He didn't fall with no resistance, isayama drew his box cutters falling off and him letting go of the grips, but the made sure to show that the hooks remained inside Berts teeth until fully extended, showing them still leaving his waist band.
4. The image shows that they are incredibly long even having not reached their full length, based on the fact that they are still scrunched towards the end and not completely straight. Do to the angle of the shot they definitely aren't a full 50 meters long, but easily appear over 20.
That being said armin likely only free fell somewhere between 10 and 20 meters, which the average person can get up and walk away from.
6. The burns he received are second degree, which cannon't actually kill anyone, all this means is the epidermis is completely gone, a person feels intense pain but it's becoming suceitible to everything that kills you, not the burns
themselves. Only 3rd degree burns(the the bone) are deadly on their own, and given he was unconscious he likely can't even remember what it felt like.
All in all, In conjunction these things would definitely kill someone IF NOT TREATED. You'd be alive in in a coma for a wealth of time, you body just preforming it's involuntary necessary actions like breathing, for a while before you completely give out.
