Aspiration pneumonia causes death because the macrophages in the lungs cannot keep up with the removal of the bacteria growing in the alveolar. The alveoli become clogged with bacteria and eventually lose all function because the lungs actually drown in the bacteria. Unfortunately, any foreign substance that gets into the lungs has the potential for causing aspirate pneumonia.
I'm so sorry about your little dog. That's a rough time for you both.
I've seen aspiration pneumonia in animals, particularly baby squirrels. People mean well and try to help by giving a baby squirrel some fluid by mouth. Problem is it takes a gentle, trained hand to feed a small animal and too often the animal will aspirate (breath in) some of the fluid, food or formula. It gets into the lungs and is a perfect medium for bacteria to grow. And it can be hard to catch it in time. By the time I see the animal at work, it's already non-responsive and having trouble breathing.
The treatment plan was excellent but it's a tough thing to beat. Again, I'm sorry.