This is just really weakly argued. Firstly, the notation does use capital X and and lowercase x. You need to realize these are different. It's not substitution

This is just really weakly argued. Firstly, the notation does use capital X and and lowercase x. You need to realize these are different. It's not substitution to go from X^2 to x^2. Capital letters are not real numbers, and thus it's a type error too. Secondly, if you are familiar with differentiation and integration, you are familiar with straight substitution not always being correct.