If this is true, one would expect vulnerability to propaganda in those forms to be going down over time, all things kept equal. Despite the cynical answer, I think that's actually true. Propaganda from, say, 1950 seems laughably crude. Some of that may be cultural messages that signal that of course propagandizing against the Nazis is silly, but some of that is, I believe, the fact that we are more hardened against that stuff both individually and as a society than we used to be. Of course, messages have coevolved with the media, so it can be hard to see the progress, but I contend that it is at least plausible that we have made progress.
Similarly, it is fashionable to believe that our education level has gone monotonically downhill for decades, but I would contend that it is likely that there are just as many idiots (proportionally) in 1909 as there are in 2009, it is just that the only surviving cultural material from 1909 are virtually by definition from the upper, educated class, whereas in 2009 the media is much more egalitarian. This has the effect of making the past look much more educated, but this can be a selection-bias induced illusion.
I think these two ideas are worth considering. I am actually very harsh on the modern educational system and broadly agree with this essay in most ways, but it is still important to separate the real problems from imagined problems, and to find the real causes and not the bias-feeding, cognitively-convenient ones. There are things that argue against these points, too, but they are worth considering.