It's not a matter of separating the lungs and intestines. As Mike has said, it's about the nerve causing the diaphragm to work failing, causing wastage and the inability of the diaphragm to function.
For many months Chris would basically just breath with accessory muscles, so a breath in was like him lifting his shoulders in a shrug. The diaphragm dropping down causes air to be drawn deeply into the lungs, and a good push up again from the diaphragm causes all that CO2 soaked air to be expelled. The intercostal muscles (the ones between the ribs) pull the lungs outwards, and squeeze them inwards to allow lots of air to fill and be expelled.
Usually we don't use the upper chest and shoulder muscles to breath, but you may notice yourself use them if you need a really deep breath or you do a big yawn - these are the accessory muscles.
The lungs are not able to move themselves. It's the diaphragm and intercostal muscles that expand and squeeze the lungs - think of a set of bellows. The main body of the bellows doesn't move itself, but widening the handles pulls air into it expanding the balloon and pushing the handles together empties it again.
Bipap is very different to muscle movement, but it forces air into the lungs, then reduces the pressure to allow air to be expelled. This is why bipap will only work while there is some muscle strength working to breath, bipap assists the breathing. An invasive vent on the other hand actually does all the breathing requiring no diaphragm or intercostal muscles to be working.
I believe that this is why bipap early is so important because if you are struggling to breathe, you are fatiguing the muscles that are already failing and breathing becomes more difficult and progression of those failing muscles may only be hastened. Let alone you are living with a CO2 soaked body which can't help anything at all.