'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.
Being fat is not incredibly healthy, on the other hand - and there is absolutely no chance that there is any confusion between the two. This isn't to prevent Timmy from thinking certain people are called lipids.