I used to own a 2.3m Dumerils Boa from Madagascar. That thing was really damn strong. I chose to give it up for adoption after I broke both of my arms and no longer felt strong enough to handle it properly.
They're strong enough that their prey can't push out of the constriction, and they'll just sit and wait for them to die from lack of blood flow then start eating.
Crushing the air out of a mammal is a lot harder, first because it just takes more force to prevent the lungs from expanding, and second because mammals can typically thrash for longer without oxygen than without blood.
Breaking any bones would take (usually) considerably more force than even that, bones are pretty strong.
They'll wait for death before letting go because snakes are pretty defenseless while eating, since their main (sometimes only) defense is their bite, which they can't use while eating.
Constrictors are also pretty good at feeling their prey's pulse while constricting as well, so they're well aware of when it's safe to let go of their prey.
Edit: This is to say that it's unlikely that the snake broke GP's arms.
That’s if you live-feed, which is generally considered unsafe. Larger snakes need larger food, and larger food tends to fight back. Yes, in the wild the snake wins almost all the time, but if the snake’s your pet you want it to win 100% of the time. Most responsible snake keepers will thaw frozen food, and most constrictor can be trained to eat dead rodents, as long as they’re warm enough.
I thought the trick used by snakes is to prevent the victim from breathing in, not forcing them to breath out (i.e. they don’t need to actively push against the victim’s expanding lung, but just statically prevent it from expanding and then tighten their grip every time the victim breaths out a bit), but apparently I’m wrong. As you say, they put so much pressure on your veins and arteries that you can’t pump blood around anymore (https://www.nationalgeographic.com/animals/article/150722-bo...)
I’ve had snakes up to 2 meters long, and even those when wrapped around my arm were not possible to force off without injuring them. I never had to do that, thankfully, just be patient (as my hand got tingly) and give them something more interesting to do.
The thing about snakes is — they’re really dumb. They have the bare minimum intelligence that a living stomach needs to crawl around and inhale things. They get food-fixated easily (and human fingers look just like mice to them) and they scare easily. Their reactions to both are the same — squeeze it until it stops moving.