“they do learn rishonim and acharonim”
I don’t think rishonim were ever in question here. Even Lev Tahor probably learns Rashi.
But, which acharonim do you think Chabad learns from outside of Lubavitch? When they say they’re learning Shulchan Aruch, they actually mean Shulchan Aruch HaRav. If you read into Milhouse’s comment, you can see that they don’t learn the classic halachic meforshim, which were all written on the real Shulchan Aruch.
Simply go to the other threads and see how often they can bring proofs from non-Lubavich acharonim: a whopping 0.