It may seem like an attack, but you can also consider it fair comment as he is an expert tell us that sugar causes weight gain, when he himself is obese for many years now. How can you both be an expert on weightloss and obese at the same time?

You're confusing knowledge and application of that knowledge in a personal situation.

A cosmologist may know all about the orbit of the planets but still say 'The sun rose this morning'.

I can think of plenty of reasons: he might not care (or care enough to change his life style to the degree necessary). He might have a disease that causes it. Or maybe he doesn't know how to get slim, but still knows that reducing fat is not the answer.