(I get what you're saying, spiritually, your pasta water from your giant pot of one box of pasta isn't gonna do much to thicken your sauce. But it's not a myth, just a matter of degree)
corn starch is widely used because it has no taste raw; a flour based roux needs to be "browned" in oil to eliminate the floury taste (i've tasted the grain of wheat from a plant in a field: tastes floury)
... if you use less water than the amount prescribed on the box it'll be proportionally starchier. It isn't a myth, you can literally see the starch in the water ...
(I get what you're saying, spiritually, your pasta water from your giant pot of one box of pasta isn't gonna do much to thicken your sauce. But it's not a myth, just a matter of degree)