The US Senate is not an agency. They are a branch of Congress, and therefore definitely not required to do anything a executive agency tells them to do -- especially if it's reasonable like implementing HTTPS.
Yes, the IPv6 is a White House (not GSA) mandate, like https://https.cio.gov. It doesn't apply to the legislative or judicial branches, and in that blog post, GSA is advertising the services it can offer other agencies to help them achieve IPv6 compliance.