The study compares humans (who happen to live in cities) to primates (who definitely don't live in cities). It doesn't compare between human populations.
For a dev with little to no time, this does basically say you can't show other sign in options. You can have only Apple sign in, but not only $competitor sign in.
Let's say I have an old app that isn't bringing me much new revenue. I have currently have implemented two options: sign in with email + password, and "sign in with Google".
After this announcement, I would just remove "sign in with Google" as an option, and force all my users who previously used "sign in with Google" to set a password.
This makes sense - my grandfather was a top lawyer in his home country before fleeing a war. As a refugee in a foreign country, he did manage to get a law position with some connections he had - but his XX years of foreign law experience weren't very useful in the new country.
Many sign up forms disallow the use of '+' in email addresses, either to prevent this, or more likely because the programmer wasn't aware it was a valid character for an email address in the first place.
What happens when a URE is encountered and all the disks are online? It seems that they could be detected early and fixed before a rebuild is necessary by doing a weekly sweep of the entire array, reading every data block.