John says he's swimming, when he is treading water.
By definition, swimming involves voluntary movement of the body.
What word or phrase can we use to tell John that he can't be swimming because he's not moving himself?
a misapplied or inappropriate name or designation.
"to call this swimming would be a misnomer."
Strictly speaking, @Okoning is quite correct in saying John's usage is a "misnomer". I will admit my first thought was a misnomer always and only means an instance of the use of a wrong name, but I see OED say it can also be used of a misapplication of a term (such as the verb "swimming").
In practice though, I think in normal conversation you'd be more likely to say...
Strictly speaking, John, you're not really "swimming", because you're not moving through the water. You're just treading water in one spot.
strictly speaking - being completely accurate (Cambridge Dictionaries Online)