I think we are both saying the same thing.
The Stromberg and the Su 'measure' the manifold pressure and adjust
the venturi to keep fuel flow consistant with air flow.
I don't know what books you are reading but fuel/air mixtures are not
constant throughout an engine's operating range.
(idle, acceleration, decceleration, etc)
However in the case of the Stromberg's and Su's that could be the case.
Maybe that's why the Stromberg's are such a pain to set and could also
explain why the 2500cc gets such lowsy gas mileage.