You may be correct about that. The last time I did in-depth testing, browsers were raft with inconsistencies when it came HTTPS caching (regardless of whether you sent the correct headers). Firefox wasn't even caching HTTPS content in memory at one point. Thorough analysis of more current browser versions would be interesting.

You've glossed over the actual point of the post though. Even if every version of every browser did respect the cache-control header for SSL content, over-referencing the SSL version of the script is still fragmenting your local cache unnecessarily. With many thousand sites already using the regular HTTP references to the Google CDN, sites that use the HTTPS reference on HTTP pages are missing out on the cross-site caching benefit, which is probably the biggest advantage of using a shared CDN to begin with.

I've been seeing more and more people using the fixed HTTPS reference on simple sites like WordPress blogs, thinking it's more secure or that it allows them the flexibility to offer their site through both HTTP and HTTPS. In reality, that's just harming their site's performance for most visitors, whereas the protocol-less reference gives them the best of both worlds. Hence the post.