I’ve made obscene amounts of home make macaroni and cheese over the last 20+ years and haven’t had a problem with it. I know it’s a funny place some people get passionate about, but the “anti-clumping agents” are typically some form of vegetable starch or fiber. If I’m making a cheese sauce I’m already using flour to help thicken and stabilize it anyway, so I don’t think the trace amounts really matter.