How the hell did I never notice Mycroft trying to suppress a little smirk in this scene (second .gif). John’s saying Sherlock doesn’t feel things in that way and Mycroft is like, “Oh John, you stupid boy. You are new.”
I LOVE that little smirk!
I’ve always thought that it means not only something directed at John and along the line you pointed out (“Oh, John, you think you know him, by now, but how far are you, still, from really understanding my little brother!”), but ALSO something directed more to Sherlock (“So, brother dear, a nice job you made in persuading even your flatmate and best friend that you are different from any other human being - that you are not even actually human - and that you are immune to such mundane sentiments and emotions… And I detect also a bit of wounded pride, behind it all, don’t I?”).
Of course, as usual, I’m probably reading too much, in it…