Good observation as always sir Yankmod, however Brooks Brothers(original "polo" button-down debuting around 1900-'02) , J. Press, Gant, and other shops were purveyors of a cohesive and established "Ivy League look" for many decades before Pendleton's contribution to it. Granted it started as the preferred dress of the patrician, blue-blooded, old money folk... But it trickled "up" to poor and working class folk like my Grandfather's generation(born in 1920).
It seems like that look took hold of Joe Public in a similar way to how it, mod, etc., influenced skinhead-- albeit at a snail's pace and not over the spam of a couple years. Probably because in America it started with conservative adults copying younger people(or continuing to dress casually in their 3/2 roll jackets, knit or repp ties, etc) as opposed to in Britain where it was a youth cult's ever-changing and fickle fashion.
Yes the Ivy shops existed prior to Pendleton but,they were out of reach to the average American who didn't go to Prep School and then on to an Ivy colleges.Pendleton made it affordable to the Masses,the Peasantry.