I already explained the difference between the gemara and the SA. The SA is working with exactly the same thing we have, i.e. the gemara and the rishonim.
And what did the Gemorah have that we do not?
(Just as you explained why you wouldn’t argue against the Gemorah [although you astonishingly maintain you can argue against the Mishna and Gemorah!!] saying “I simply think it is unwise, very unwise, being that the amoraim knew a lot more than us, whether with regard to obscure braisos, or kabalos from their rabbeim since Moshe.”, the mechaber too knew a lot more than us, a better understanding of the mishna and gemorahs, and including a closer line of kabolos from rebbeim through Moshe Rabbeinu. — This point, again, being made running along your stated line of thinking.)