Yes, as long as that person is also considered "among us".
Edit - we don't know for sure that John didn't take a breath. In fact you'd often assume that this was an extra hindrance that the new person had that John didn't. "I never thought Peter would swim as fast as John, especially with one leg". Here not only has Peter matched John but he did it with only one leg whereas John presumably had two.
Yes, I presume in your mind you had something like "especially since to do that you have to hold your breath the whole way (which most people couldn't)", or similar - which is also a reasonable interpretation, just not the only one.
The idea was that someone else goes as fast as John (and that is a surprise) and on top of that, just like John, that person doesn't breathe when he or she is doing it. So I thought it was impossible to go as fast as him, and it was even more impossible not to breathe while one was doing it (just as he doesn't). The 'especially' provides a handy short cut.