The claim that Ukraine is the (exclusive) successor of Kievan Rus is something of a stretch. There is a reason that Chlemnitzky, not the leaders of Rus, is the Ukrainian nationalist hero and is considered the founding father of Ukraine
This is a quote from the video, from Henry Abramson, who has a PhD in Ukrainian Studies: "...it would be more appropriate to say that Russia belongs to Ukraine than to say that Ukraine belongs to Russia."
Khmelnytsky is definitely a nationalist hero, but that doesn't make him a 'founder'. You yourself pointed out that under Khmelnytsky they just went from being under the Polish to the Russians. That doesn't preclude someone saying they had a prior history. Your point about Khmelnytsky proves absolutely nothing. That's like saying that because Ben Gurion is considered a founding father of the modern state of Israel, therefore there was never an independent Israel in the past.