The claim here is that vegan athletes require supplementation, therefore there is something wrong with veganism
Again, this isn't you. You can't justify needlessly killing animals because some athlete somewhere has to take a supplement.
So this can't be viewed as some kind of negative against veganism when carnists do it as well.
Even if vegans did and carnists didn't supplement, you can't justify needlessly killing animals by saying that you're avoiding reaching for a box of protein powder or some vitamins, specifically if you can still get those nutrients from the supplement.