It's actually a hcaptcha: https://i.imgur.com/JcStn1t.png

Also parent post about "requiring a phone number" or "giving them another email" is incorrect, you can choose to supply one if you want.

Not everyone on the internet has a requirement on being anonymous, so it makes perfect sense.

On a side note, you can fill out a captcha and remain anonymous, also they also have a v3 .onion unlike a lot of providers: https://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7...