'Fat' should not be used in kids books as a derogatory, or at all frankly. Fat is an incredibly important and necessary part of a healthy diet and should be treared accordingly. Training people to think of fat in the antiquated notions of bad cardiogy isnt useful.
So while this may have been a move centered around "body positivity", it serves and higher purpose.
You will not locate a doctor on this entire planet who would agree that obesity is healthy, it places far more load on the joints, and puts additional stress on your organs.
Either you have a deeply profound misunderstanding of the adjective fat versus the noun fat, or you're deliberately misconstruing the authors intent to fabricate an argument for why censorship is OK.