I'm not sure where you're getting this. Of course the Founders understood that people were subject to the influence of others; again, this is just basic human psychology, something they accounted for in their writings. For instance, Thomas Jefferson said:
Nothing can now be believed which is seen in a newspaper. Truth itself becomes suspicious by being put into that polluted vehicle. The real extent of this state of misinformation is known only to those who are in situations to confront facts within their knowledge with the lies of the day.
As you can plainly see, Thomas Jefferson understood quite well that people are subject to the ill influence of sophisticated and immoral people. This is just one example of literally thousands that I could show you, demonstrating the Founder' ability to perceive exactly what you say they cannot.
Perhaps you should, I dunno, sit down and read them at length, instead of trying to encapsulate the culmination of hundreds of years of political and ethical philosophy into these convenient soundbites. I'm not trying to be rude, but you don't seem to understand their "theories" very well.
Also, where in the world did you get the idea that "propaganda" (which is nothing more than the deliberate 'propagation' of information or ideology) is only a 100 years old!? For as long as ideas have existed, humans have sought to propagate them; this is certainly not a recent phenomenon.