It didn't sound like arrogance to me. The "if" at the start of the sentence indicates they are not asserting their conclusion as the only truth. That said, if toxicity is a large concern, it's strange that companies sell such high dosages at least without some kind of warning. I bet a sufficient warning would prevent at least some cases of toxicity
A lot of OTC medications have potential issues and interactions. That said, my primary care recommended I take vitamin D based on blood work and no one has batted an eye at my taking 5K IU. So the idea that's some widely-accepted radically dangerous dose is simply untrue. (Doesn't mean it's a wrong idea but it's not one that doctors generally accept.)