The article doesn't link the original court filing or discuss what actually happened, and from the title alone, is rather misleading.

The actual warrant can be found here and has the important missing details: https://drive.proton.me/urls/57QC5F26BW#nseYl6ICaQHm

The only data we could provide (in response to a binding Swiss legal order), was the user's recovery email address, which the user added himself, and is optional to begin with.

Unfortunately, said user also used that recovery address to create a Twitter account, and Twitter turned over his phone number and IP address.

Coincidentally, this case again proves that Proton Mail's encryption cannot be bypassed by law enforcement.