News Volume 96 coming on April 2020

Koshiro being from the Shimotsuki clan does not mean Zoro isn't a descendant from the same clan. Kozaburo was not the only person to leave Wano.
But Zoro has heard the name Shimotsuki multiple times now and doesn't care. So you would have to say Zoro doesnt know he's from that clan for SOME REASON. Then after that you would have to explain who Koushiro never mentions that he's related to Zoro for SOME REASON