Netherlands is currently at 30000 cases with 3000 dead, cfr of 10%. 20x infection would put the IFR at 0.5% which is well in line with what is expected

Possibly dumb question, how do we know someone with antibodies was exposed? The whole idea of vaccination is based on multiple things producing the same antibodies, so isn't it plausible there could be related viruses or genetic factors or something that would lead to antibodies for some people that hadn't been exposed?

Yes, there is the chance of false positives with antibody tests, in particular for other strains of Corona, but scientists can do a sample of antibody tests on people they know to have been infected with covid-19 and get a rate for false positives and take that into account.