The subtext sounds like "we guarantee your returns, then go public. If we go bankrupt you get the retail investors' money, if we become the next Google you get your own private island." All you have to do is trust Sam Altman and (breaks out in hysterical laughter).
Do they mean 17.5% a year? My balanced bond-equity portfolio made 14-15% annual returns over the past three years by the radical method of buying "shares of companies that make profits" and "bonds backed by my local and national government." (Update: I made about 12% a year because I backed out of American stocks years ago, but the blandest 60% stock, 40% bond index fund in my country returned that 14-15% a year after expenses).