The actual commits will have the fake address. The change isn't that they're hiding the email addresses in the UI; it's that they've added the option to use a fake email address for commits made via the web UI while still receiving notification emails from Github.
You don't need a valid email to commit to your repositories, but you do need one to use GitHub notifications. This feature makes it possible to use the web editor with a fake email address instead of your notifications email address.
Absolutely nothing. According to the linked help page, a fake email address must be provided to git config. They'd need a separate copy of the repo with the blinded email address for public access. It's not hard to do but I imagine it would bother repo maintainers that an individual user could cause different repos to be served. Of course, everyone could just decide that email in commits isn't worth the privacy risk. It's a trade-off.
For the curious: Downloading all the repos and extracting email addresses takes 1-2 days and costs <$50 on AWS.