The figurative and derogatory meaning of 'back-seat driver' is unambiguous in this from The Bismarck Tribune a few years later - December 1921:
"A back-seat driver is the pest who sits on the rear cushions of a motor car and tells the driver what to do. He issues a lot of instructions, gives a lot of advice, offers no end of criticism. And doesn't do a bit of work." (source)
Today it means someone (or something) basically doing the same thing but in more figurative terms.