No surprise to me. My mom died of Alzheimer's, and I lived with her for the last part of her life, so I tried to learn everything I could about it. There are a few reasons why it's hard to keep people with dementia fed. Early on, they simple forget to eat or think they have. Before I lived with her, I'd visit my mom and ask if she was hungry, and often she'd say no, she just ate. After I moved in it became clear that she'd say that even if she hadn't eaten all day - she was remembering some past meal.

Later in the disease, people start having a hard time swallowing solids. My mom got so that she was practically living on Ensure because she could drink from a straw but had trouble with anything more solid than pudding.

My mom had someone attending to her, watching over her constantly, but certainly not every dementia patient does. Most people with dementia die of secondary things because they choke on food, fall, have issues from being bed bound, get pneumonia, etc., and being under nourished makes a lot of that worse (and can be fatal in itself). My mom was never heavy, and she died because her heart just stopped, but I'm not surprised that people who start off heavy do better for longer.