They literally guarantee you making over 300% per year off your investment, and I fail to see how that can be done without running a ponzi scheme of some sort.
Only Ponzis can claim Guaranteed returns, because they know that won't be paying it anyway. The OP is correct, the argument that some tokens were bought back doesn't hold water as it might be simply to keep the Ponzi running and to get more investors. It's funny how people continue to get scammed in these schemes; I personally know a few people who were burned in BitConnect and still continue to invest in these. Some people never learn.