You're not necessarily wrong about insulin responding to everything, but fiber definitely plays a pivotal role in regulating the deployment of insulin in response to how much sugar your digestive track actually absorbs because of how it physically coats the inside of your digestive track during the digestive process. If you don't eat something fibrous before the sugar on your plate, you risk oxidative stress, free radical production, neuropathy, vascular damage, and chronic inflammation of the body because of the insulin response to unmoderated glucose spikes.

Similar to how vegans unrealistically expect everyone to stick to a straight plant based diet and dismiss qualities of other foods, advising people that fiber is useless is not really setting them on a path to heath. It's about moderating your blood glucose spikes to maintain a cool balance in your body.