My first reading of "by default" here is that I can optionally enable it through my account.
Really, it's a phrase that means 3 things: I can enable it, ProtonMail can enable it[0], or the authorities can compel ProtonMail to enable it.
Saying any of that, or at least linking to a page that does, would be a smart move.
[0] https://protonmail.com/privacy-policy - "IP logs may be kept temporarily to combat abuse and fraud, and your IP address may be retained permanently if you are engaged in activities that breach our terms and conditions"
