I can't resist... Language is, in fact, a matter of common usage. A man is (or was) generally considered to the thing that is (or was) a male human. While I recognise that gender theory has challenged this notion, introducing the idea that gender is socially constructed, it can easily be understood (by those who have the intention of understanding) why this confusion might exist.
Before contemporary gender theory challenged the distinction between male and man, there was also a common distinction, e.g. by "becoming a man". A man is a male human adult, and the idea of adulthood is to some degree fluid. The legal age limit of 18 or 21 suggests that this is/was when we consider humans adults. But notions of "when your brain fully develops" suggests ages of 25-28 for men. Other factors involve when your beard grows big enough, and when children on the sidewalk saying "that man" about you. Society seems to have a duck-typing approach to what a man is, i.e. when a male human transitions from boy to man. And indeed, transgender men fit the duck-typing of society to a point where "strongly influenced by testosterone" seems like just as good a definition of, at least, "manly". Contemporary Western culture would have that you can make all sorts of other transitions at different points.