Earlier quoted context omitted.
An expired certificate still encrypts your traffic. You might have to change settings or click through a scary warning in your browser, but other than that a certificate doesn't magically quit working as soon as it expires. The expiration date is arbitrary.
You are correct, I had to do a bit of research. Because Chrome even explicitly states that traffic to a site with an expired certificate is unencrypted. But I guess that's mostly to scare you, because the truth is that it just opens you up to potential MitM attacks and other similar issues with regular ole HTTP, but traffic between you and an unverifiable identity is at least TLS encrypted.
If that's the case, then Google's condescension is doing a disservice to its users.