'Primary email address' would be emailaddress1, however once the Contact record is associated with an External Identity Provider (such as B2C) the link between the Identity Provider and the Contact record is the identifier from the Identity Provider (in the case of B2C this is the guid of the B2C user) this linkage is stored in the External Identity table which is linked to the Contact (it is not the email address of the Contact).
Note: (except for local authentication) you are not really logging in to Power Pages, you are logging into the Identity Provider, and Power Pages is trusting that Identity Provider.
Where this is being described in the article is primarily about moving a Contact that was setup using Local Authentication and then moving to B2C (but part would also apply to where you have an existing Contact created and no authentication setup).
Where you are referring to in the article you linked to unfortunately is not 100% clear on what they are doing. The scenario in the article is relating to having an existing Contact and having it map to the B2C when the users B2C is originally setup - this is also not the default behavior and requires a setting on the identity provider setup in Power Pages see "contact mapping with email"