No it isn't. That's flat nonsense.
GI still gives you an indicator of the overall effect on blood sugar.
Let's say you had a sample meal of chicken, avocado, broccoli... and a 100g of a carb source.
That carb could be:
A) beans
B) pasta
C) White rice
D) Bowl of sugar
Obviously, the sugar or white rice is going to give a larger insulin and blood sugar response than the other 2.
If taken in ISOLATION, the sugar will give a greater blood sugar spike than the other carbs, if they are also taken in isolation.
When eaten with a mixed meal, the proteins and fats are going to blunt the blood sugar response from any added carbs. However, you should not interpret that to meal that your carb source or GI are irrelevant.
chicken, avocado, broccoli, and pasta will give a lower total GI than chicken, avocado, broccoli, and sugar.