To add a little to Jim's excellent answer.

Yes airfoils with higher maximum lift coefficients will slow down further before stalling. That's one item that should have been in that formula. If you go back and read the article you may find that they assumed a max Cl of 1 or some other amount. Cl=0.9 to 1.2 is quite a common max lift coefficient range for model type airfoils so assuming the value equals 1 isn't that far out.

And flat bottomed airfoils do not have a built in angle of attack. Folks just draw them that way (like Jessica Rabbit ). Properley presented a ClarkY should be drawn with the chord line from the very peak of the leading edge to the point on the trailing edge in horizontal alignment. When done that way the flat lower surface will be angled. Don't be confused by common convention even if it IS common.