Earlier quoted context omitted.
Fat is an essential macronutrient.
Fat includes transfat. This back-and-forth does an excellent job of underlining GPs point about those being complex subjects that require study and nuanced explanation.
There is hardly a back and forth here. Fat is essential and a lot of is not harmful.
The best you could do is to point out that heavily processed vegetable oils are harmful and probably give you too much omega-6. The cure? Just switch to things like olive oil and animal fats.
Compare this to sugar (also mentioned by the poster).. Which is... not essential to anything. Your body does not need it. It is actively harmful. Carbohydrates however can be fine, and fiber is good, but the particular kind of carbohydrate known as sugar (like the two molecular compounds more or less) are no bueno.