Mencken really goofed on this topic. He gives US "typewriter (operator)" as equivalent of UK "typist".
I have never heard "typewriter" used to mean the operator thereof. It is given as second meaning in Webster 1913.
Source: Webster's Revised Unabridged Dictionary (1913)

Typewriter \Type"writ`er\, n.
1. An instrument for writing by means of type, a typewheel,
or the like, in which the operator makes use of a sort of
keyboard, in order to obtain printed impressions of the
characters upon paper.

2. One who uses such an instrument.