You can’t exactly “revert” an anonymized user without restoring from a backup (which also reverts anything else that happened since the backup took place).
However, assuming you can find the account in question, anonymous accounts can be logged in to so long as there is a password set.
If a password was not set, you’ll have to change the email address either via impersonation or via the rails console, and have them sign in with a social account under the same email or via link (or password reset).