Not true, at all. For DC generators, all that matters is how fast the wires move under the pole pieces. Slower prime movers just mean that the generators have to have a larger radius. I have seen pictures of early generators that were several feet across, with what looked like a spoked wheel for a rotor. Not that the upper speed of a reciprocating steam engine was an issue, as a steam locomotive had no difficulty hitting a hundred miles an hour on straight level track with a short enough rebound time.
The really wide generators were needed for hydraulic turbines, as they were quite large and only spun at about 60 to 240 rpm (one to four hertz).
AC generators deal with a low mechanical frequency by multplying the mechanical frequency by a large number of pole pairs to get a useful electrical frequency. I have seen photos of small angle segments of large generators for hydraulic turbines that implied more than thirty poles (my father gave me his copy of the Westinghouse Handbook of Generation and Distribution, and most of its information is still valid[my copy is dated
1950]).There were two reasons steam turbines blew away reciprocating steam engines: Turbines and their generators were smaller Turbines shook less and their vibrations did not carry as far