Strong John.
I feel like a lot of women don’t really understand how strong men are, on average. Even men who aren’t all that well-developed can generally lift the weight of an average full-grown person well enough to potato-sack a drunk friend into bed.
John and Sherlock are both in good enough shape to half-ass a parkour run across London’s rooftops, so either of them probably wouldn’t have trouble slinging the other over his shoulder and hauling him up the stairs.
Part of it is just an issue of size. I’ve seen taller women manhandle male friends around like this too.
(Now, if you want a notable feat of strength, Sherlock’s apparently fit enough to free-climb the outside of a hospital building with a bullet wound in his abdomen.)
Somehow I don’t think he did the window trick. Otherwise why do we see a wheelchair complete with IV stand and bag of fluids at the end of the hall in the empty house where John sat during the conversation with Mary? I surmise he called Billy to assist in his escape. Billy could have dressed in scrubs and wheeled him out under the noses of the entire staff. Nothing unusual in that. Also he had to go to Baker Street and move John’s chair, place the perfume and acquire clothes. Just saying.
An excellent point!