Even worse:

The identity and location of the activist was already known to the French authorities (they had already been evicted once before for squatting, and the nature of squatting means that their location is known).

So they were probably not using a VPN to connect to Proton Mail, which was the specific target, since e-mail and VPN providers were treated differently under Swiss law until Proton and Threema fought the government on this issue. Tutanota had a similar issue. If you're gonna rely on these services to break their jurisdiction's laws, you should be covering your own ass with bulletproof opsec, because businesses with millions of accounts are not gonna shut down and burn evidence in order to protect one user. In the Proton case, the activist apparently connected to a known Proton Mail account with no VPN or Tor; in the Tutanota case, only e-mails that were not end-to-end encrypted would pose at risk