The easier way is for FB to wait until the GP friends the friends, and with some or many of them having already synced their contacts/address book with FB, tie

The easier way is for FB to wait until the GP friends the friends, and with some or many of them having already synced their contacts/address book with FB, tie the GP back to the old ("deleted") identity using the phone number. This is a simplistic explanation, but there are more ways to identify people based on the name and other attributes one's "friends" upload on to FB. In effect, what the GP does in no way helps provide more privacy from FB for the GP.