The problem is determining what from that period constitutes “Ukrainian” or “Russian”. The old “Rus peoples” were a lot of relatively different yet also similar groups, that homogenize and heterogenize across time depending on different political and social conditions, which at a certain point in time during the modern age some intellectuals try to categorize and end up reaching some sort of consensus with “Russian”, “Byelorussian” and “Ukrainian”.
In the end, forgetting this kind of nuance invites people who are a little shaky on the head to come up with pan-nationalist narratives that justify some sort of anachronistic imperialism. It’s already bad enough that Putin’s Z-morons are trying to appropriate Ukrainian identity as a breakaway of their own to justify ethnic cleansing, let’s not have some Ukrainians that try and do the same but in reverse.