The debt wasn't being paid off at a great rate but it was being paid off and the person's main point that you responded to is correct. We were well off financially until some very irresponsible actions by the Bush administration.
I disagree with the point about us never being able to pay off the debt. It can be done and there are historical examples of countries with higher debt/GDP ratios that were paid off.
Note: When talking about debt of a country people generally mean the debt of the government. There was a budget surplus at the end of the Clinton presidency and this is what people usually mean when they claim a government was/is "paying off the debt".