I think that American golf architecture starting with the National and up till the early 30s was into some new and different ideas based upon the natural look of links courses in Scotland but also on the efforts of Colt and others in the heathlands around London-- modern or scientific designs if you will.
Not just the designs but in the way golf courses were constructed and priced. Engineers were partnering with designers and in the case of Flynn and his engineer Howard Toomey, they were following the principals of time management and efficiency that Taylor (not the Br player and architect--Tom Paul, what was his name--Frederick?). Flynn knew, after building hundreds of greens on all kinds of different terrains, the average cost of a green on a given type of terrain and priced his contracts based upon this.
Flynn developed a style of using natural lines even on sites that required significant engineering (Cascades, Indian Creek, etc.). He knew this cost more money to build initially due to the increased fill and time to build natural lines around man-made features. He and Toomey devised a scheme to address the added cost and knew it in advance to inform clients. The move away from the geometric in America and to a naturalism was a much more appealing look and was cheaper to maintain in the long run, also proven by Flynn and Toomey's studies.
As far as designs, it is simplistic to think that Flynn ushered in the principal of fairness in golf because he had a propensity to use flashed bunkers and didn't often employ concealed bunkers. I agree with Tom and Tom that it was happening elsewhere and before Flynn.
Flynn also was an early an often user of multiple tees as well...this is probably a stronger case for your argument, Pat.
What was considered modern were concepts such as interrupted fairways such as those at Pine Valley. Flynn used these on many of the courses he designed in the early to mid 20s such as Kittansett (still there to this day), Yorktown CC where they were advertised as of the modern style, Cherry Hills, Denver CC, Atlantic City CC, Boca Raton, Opa Locka, and others. A more sophisticated use of wind, courses within courses, and reversible courses are all examples of the American design principals that Flynn was employing.