Exactly.
The non profit is registered in a small town of Maine-et-Loire.[1] Assuming this is close to where the developer lives, €42k goes a long way there. You also have to factor in that he doesn't have to take the Parisian Metro every morning and live in a tiny apartment for €1,000/month.
[1] See the "Is Lichess a non profit?" https://lichess.org/patron
Note that a competent full-stack Scala developer like Thibault can easily make €70-80k at a French company while working mostly/fully remote. And even more so going into contracting and/or working for foreign clients.
He has his own reasons, working on his own thing being a huge one obviously, but he could definitely make more and keep a similar lifestyle.
Personally I would struggle to pay for a house, support dependants, and save for retirement on that amount. Partially because I went back to $zero in my thirties (although I know plenty of people in the same category much older... Trying to catch up is hard mode).
[π] https://old.reddit.com/r/chess/comments/mpasyl/i_started_lic...
Not sure how companies advertising take-home pay would work when your tax rate depends on e.g. marital status (at least in Germany it could be off by maybe a factor of 2 in both directions), which country/region are you thinking of there?
The one I used to work for had a real problem of developers leaving for the US on an internal transfer after 12 months, precisely because of the salaries.
It might sound weirdly low for someone in IT in the USA, but first, not everyone commands FAANG sales, and second, wages in the EU are comparatively low. With some seniority, you can rise to $100k relatively easily, but you have to sell your soul to either some consulting company or old Fortune-500 industries.
So could be the dev takes an above-avg wage which would be around junior to semi experienced level developer and just does what they want to do: build a chess community.
EDIT: I don't fully understand the tax part, but if the nonprofit also pays his taxes and he goes out with $54k net, he's doing very, very well for an EU country. I've just reached that level give or take and I have lots of seniority in a really profitable old economy country. If I would be gunning higher, I'd have to do consulting or go the people management track.
[1] https://en.wikipedia.org/wiki/List_of_European_countries_by_...