It was probably the 2017 model, it had a fan. But it was in the tight pouch within the backpack that was supposed to protect laptops from bumps, so it didn't have much room to breath.

Same thing happened to me with my Surface Book. I found it the next day with fans at max and burning hot. Still survived. Why aren't laptops shutting down when the CPU gets unbearably hot?

"Unbearably hot" for meatbags and "unbearably hot" for CPUs are different things; meatbags start having troubles at 50°C, while internal components are fine until 90-120°C.

Traditional hard disks are the biggest exception, but I'm fairly certain that's not a worry for a Surface.

Good question, they do turn off the discrete GPU when it gets too hot.

How can a fan help when fresh air is nowhere for it to suck from? I even suspect fan-less models may be better prepared for such scenario.

Even when there's not much fresh air supply, it can circulate the existing air over a larger volume and transfer heat faster into whatever material surrounds it. Doesn't help much, but might be the difference between "it's burning my fingers but still works" and "it died".