Yep, NO flaw found here, either, and I use calculus in my job as an engineer. Who thinks dy/dx is a fraction?
Agreed, I never thought of it that way, just viewed it as a notational convention. You can argue that Δy/Δx is in fact a fraction and that reducing that to the infinitesimal dy/dx doesn’t change the nature of it, but in real life nobody I know thinks of it or uses it that way.