As flawed as this is, it's in line with other studies around the world. You can nitpick and critique each one for something, but we now have a whole body of evidence using different techniques and different methods that are all stating the number of cases is vastly undercounted, and the IFR is under 1%.
Scotland: https://www.medrxiv.org/content/10.1101/2020.04.13.20060467v...
NYC Pregnant Women: https://www.nejm.org/doi/full/10.1056/NEJMc2009316?query=C19...
Finland: https://thl.fi/en/web/thlfi-en/-/number-of-people-with-coron...
Germany: https://www.land.nrw/sites/default/files/asset/document/zwis...
Chelsea, Mass.: https://www.bostonglobe.com/2020/04/17/business/nearly-third...