Based on this statement it looks like they were compelled by the courts:

In this case, Proton received a legally binding order from Swiss authorities which we are obligated to comply with. There was no possibility to appeal this particular request.

They also provided an update:

In October 2021, Proton won a Swiss court ruling that email services are not telecommunications providers. Consequently, email services are not subject to the data retention requirements imposed on telecommunications providers and are exempted from handing over certain user data in response to Swiss legal orders.

So it sounds like they had their hand forced in that instance to provide the data and got a court ruling that allows them in the future to not retain that data. I would trust them.