You asked the question, with the context "why are people bringing this up, did Trump even claim that he didn't have any business in Russia?", then, when shown that Trump did, in fact say that specifically and repeatedly, you dismiss it with "well who cares?". Why ask the question if your response was going to be "who cares?" anyway?
Having debt to Russia itself isn't impeachable. Not disclosing that debt is certainly a problem. Actions in office that might have been done for personal benefit in the context of that debt could absolutely be impeachable offenses.
He's been cozying up to Putin since his time in his campaign, with actions including, but not limited to, publicly taking Putin's word over our intelligence agencies regarding Russian collusion. If it turns out he was indebted to Russia while he was taking Russia's claims over our intelligence agencies claims, well, that might indicate that his allegiance was compromised.