@Alexandra316,
It does not matter if guest has setup a wrong private email adress, as communication within the Airbnb message system is allways possible, so also no need to ask for phone contact by the guest..
Only the forwarding of the AIrbnb messages to the private emaill address will fail (And so the private emailaddress can not be used to sent messages to the temporary Airbnb emailaddress provided).
It is most likely the guest wants something to discuss which they do not want to be recorded in the message system. BTW Exchanging phonenumbers before a reservation is confirmed is also, without "tricks", not possible (it will be blocked in the messages)
.
As what i remember i had to confirm my private emailaddress, when creating my account, in the way you describe, which are normal procedures.
Best regards,
Emiel