I don't really see benefits for fighting something which isn't (meant to be) a big thing...
Using href="//cdn.*" is the best way to use with modern browsers, if it is a need! I recommended that to use and it was (patched) committet, trusting YL's Knowledge. And the (cores) local approach is even better, since you don't talk with other servers, if that is not a ressource question. These (now) changed plugins did not have that, because they where old, I presume. Since Garvin changed them to https and YL removing the protocol, without any further discussion, I assume, we do not care about browsers being unable to read this properly, but this is another question... (a new one, that you now brought in - and I can not say, if that breaks the Serendipity compat rule...).
Back to my CDN question. But since Google serves and offers both (http and https) it is just the (my!) question, why then there is a need to use https for libraries, in general, if the first is in action. If I am wrong with that, I will stricly behave quite any further.
The mentioned blog post does not handle my origin question and was just handed by to give some other points to think about, handling cdn request over https. The google page did not talk about it, except recommending to use protocol less references.
And as you see by any follow up answers, it is not even worth taking to much notice about. It was just a thing I for myself wanted to know... why https is a need for google cdn libs here (and I did not mean because of an error, which wasn't the issue with it).
If your misleading answer links to my "So why should we need to do this via https?" question (btw, the real one is underneath), this was a question regarding the originial pasted code, which is a CDN Request. I have to admit, that I did not think about changing that core protocol request too until now, while I could not imagine someone could run a blog on https. But this might also be. So you are right on this not asked question.