I couldn't find any detailed write up about their selection methods for participants, only the basics of where they found them. Without that, it is very hard to determine whether or not there's some flavor of selection bias: If it is voluntary, and they do not screen out people who report having had cold/flu symptoms, then they run the risk of attracting a disproportionate number of people who volunteer because they'…
Do you have a source for the CFR of 0.1 for H1N1? I could only find this[1], which is a much smaller number. [1] http://news.bbc.co.uk/2/hi/health/8406723.stm
Here's my source [0] and the relevant quote, below. It is the CFR during the H1N1 pandemic, 10 weeks into it, which is why it is a much more appropriate (though not perfect) "apples to apples" comparison with Covid-19:
>"The overall case fatality rate as of 16 July 2009 (10 weeks after the first international alert) with pandemic H1N1 influenza varied from 0.1% to 5.1% depending on the country."
[0] https://www.cebm.net/covid-19/global-covid-19-case-fatality-...