You seem to be mixing two things. Paskening from only the Shulchan Aruch is Halomed Mitoch Hamishna, as the Maharal explains. You must know the reason in order to be Medameh Milsa Lamilsa. You are adding to this that it wasn’t meant as a Psak. This is not the case. Perhaps we didn’t have to Pasken like the Mechaber at the time, but many have said that Klal Yisroel accepted it as their Psak.
This actually follows the reasoning of the Beis Yosef as to why he usually Paskens from the Rif, Rambam and the Rosh. There might have been many that were equal or perhaps greater, but these took the pains to write Halacha.
We find that it works this way in Shas as well. We Pasken like Shmuel in Dinim. If the reason would be that he knows better then why would Rav argue? The idea is that we follow the one who was practicing. This is why some people Pasken like the Aruch Hashulchan over the Mishna Berura, since the former was a practicing Rav.