The best way to learn about the incident is to read the discussions first-hand from the Hacker News, for example, by searching "inurl:ycombinator protonvpn tesonet". There is no point in reading any journalistic articles if you can read Proton's responses here, except one article [0] - a compilation of changing Proton's responses and them successively admitting more and more things not in their favor. The compilation starts at the part called "Online accusations fly".
But you don't have to just take our word on it. ProtonVPN in particularly has been heavily scrutinized, by both Mozilla (who we partnered with) and also the European Commission (which is providing funding): https://protonvpn.com/blog/is-protonvpn-trustworthy/
In other words, there are plenty of non-anonymous, legitimate third party sources, who have checked things out and confirmed the story is bogus.
One main allegation was that Proton shares an address with another company, but it fails to mention that our office in Vilnius is in a 30 story office building with hundreds of other companies: https://www.instagram.com/p/BxMz62oHb6K/
[0] https://restoreprivacy.com/lawsuit-names-nordvpn-tesonet/