This debate is age old. It's not a stupid comment DrewChan. When you really ask the question what a supplement in bodybuilding there are 2-ish schools of thought
In school number 1 supplements are generally products manufactured or developed with the intent for consumers to use to better their health (this is the more broad overarching definition) - Most things fall into this category, whey, multi-vitamins, raw eggs, creatine.
In school number 2 supplements are generally low calorie products which have been manufactured with the goal to alter or improve some form of bodily function. creatine, cla, bcaas, etc fall into this category. You could say number 2 is a subset of number 1 but the philosophy and reasoning between the two schools of thought differ.
There is tonnes of literature on this discussion and to call my comment silly is rather silly in itself. What is a supplement I ask you?
When i say supplementary to your diet I was basically assuming school of thought number 2. I don't consider eggs and milk supplements since they are food substances and hold nutritional value which forms part of your diet instead of (ie is primary to your diet) instead of something else like creatine with is supplementary to your diet.