Because the guest option uses a Google Services API so the are getting your Google Account info to make the connection the Foss versions rightfully strip play services integration like most if not all foss apps.
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I see. So they stole my device's Android email address? Very disappointing. Thank you for the help.
I mean you should expect this from any play store version of a app.
You are right. Guess I expected more from a privacy advocate. Oh well. Will have to sign up with a Tuta email address (in an attempt to silo my VPN activity from my other Proton ID and usage).
I looked into this because I thought that you must be missing something, but yes, you are correct.
Both the F-Droid package as well as the one on the Proton's GitHub page do not allow for guest logins. This is curious. Perhaps the guest account option requires some sort of non-free library? I'd be interested in the answer to your question.