They do mention it on that page:

However, if presented with a valid order from a Swiss court involving a case of criminal activity that is against Swiss law, Proton Mail can be compelled to share account metadata (but not message contents or attachments) with law enforcement.

The only ever claim to encrypt message contents and attachments. And explicitly call out account meta data here as something they can hand over if requested by law enforcement. They also mention they are not good vs targeted and governmental level attacks:

There are, however, some risks for users facing a strong adversary, such as a government focusing all its resources on a very specific target.

And explicitly mention they might be compelled to log and give up information like ip adresses:

if you are breaking Swiss law, a law-abiding company such as Proton Mail can be legally compelled to log your IP address.