I don't think it is the CSS.
Whenever legacy add to cart is activated, the button is contained within the last cell of the table in "product / quantity.php"
Whenever I turn off legacy mode, the button relocates to outside of the table. Is this not something to do with the PHP in the "product / quantity.php"
Also, in the link that you posted, the information says that "In the edition page, you can find all the different parts between the HTML comments."
I am going to Display > Views > relevant file but I do not see HTML comments in any of the files which I look at?