The logic in this script comes from the second part of the tutorial link.
How can I change the author name / email of a commit?
See also SO question:
Could I change my name and surname in all previous commits?
An answer there provides a solution with a snippet. But, a comment for the answer links to an improved script:
This script is limited to replace the email address, while the Git user’s name is left unaffected. See the links above for how to set that too.
See Authors guide to change one commit at a time.