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I think you're right.

http://www.paulirish.com/2010/the-protocol-relative-url/

This page outlines the original argument, as well as the updated reasoning that you suggest: always use HTTPS if it is available, even if requesting from a page served over HTTP.

My best guess would be that any work done by a Googler will be served up from a domain included in the HSTS Preload List (forces SSL in most browsers)

I always omit the protocol because I develop on a local server using HTTP, but deploy to a remote server using HTTPS.

I agree, the protocol part seems pretty outdated.