The numbers that are quoted for Austria (which has the listed IFR of 1.66% in the document) weren't obtained via antibody tests, but via PCR tests. Here's a better source than the one linked in the document: https://www.sora.at/uploads/media/Austria_COVID-19_Prevalenc...
Most of those tests were done on April 4th and 5th which was 3 weeks after Austria started relatively strict lockdown measures, which also impacts that number, as this will result in the test to find an even lower number of positive people.
Most of those tests were done on April 4th and 5th which was 3 weeks after Austria started relatively strict lockdown measures, which also impacts that number, as this will result in the test to find an even lower number of positive people.