This is months old and was all over reddit and Lemmy. Basically the days handed over complied with Swiss law, not US iirc. So it was minimal at best.

Actually nvm the article explains it clearly,

Thanks to Proton Mail, they were able to get that information, though it's fair to note that this was a somewhat convoluted process. In order to get the information, the FBI used a Mutual Legal Assistance Treaty, or MLAT, to ask the Swiss justice department to request the information from Proton Mail. As Proton is based in Switzerland, it had to give the requested information to the Swiss authorities, who then passed it on to the FBI.

"We want to first clarify that Proton did not provide any information to the FBI, the information was obtained from the Swiss justice department via MLAT," said Proton AG's head of communications, Edward Shone. "Proton only provides the limited information that we have when issued with a legally binding order from Swiss authorities, which can only happen after all Swiss legal checks are passed. This is an important distinction because Proton operates exclusively under Swiss law."

Because the Proton Mail account was paid for by credit card, Proton AG was able to provide the payment identifier, which could then be used to find the card's holder, leading to an arrest for alleged trespassing. However, 404 Media does not name the person in question as they do not seem to have been charged since.