I think the opening of the article is unfortunate. It doesn't relate that strongly, or support the bulk of the article, which is a skim through different Signal (which as you say was a Nazi magazine).
The rest of the article is better, but still kind of surface level I feel. There are two broad strands of thought in the rest of the article: a) Signal critiques of America could be "surprisingly" nuanced and/or still resonating today and b) the tone of signal critiques show a surprising acceptance of some level of greatness of America.
I say surface level, because a) seems kind of obvious. The best propaganda is to selectively choose (and perhaps mildly exaggerate) your opponents true weaknesses/downsides, while emphasizing (or perhaps fully fabricating) your own strengths. Especially when your target is notionally the land of freedom.
For b) that's also not completely surprising. It was simply NOT feasible not acknowledge America's material strength and wealth. You could not have plausible propaganda claiming that America did not have admirable resources or industry.
Fundamentally this boils down to, Nazis were not cartoon caricatures of humans. They were humans like us, spanning the entire spectrum of temperaments, "high browness", education, and tendencies. Even more, they were humans raised in the western tradition - there were Nazis who were ever bit as informed by enlightenment thinking and greek rhetoric as we are here today. And therefore they could choose to write propaganda that would appeal or at least resonate with us to this day.
The final line however is probably the most interesting bit: "What if our enemies know us better than we know ourselves?". This line of thinking invites us to ask all sorts of interesting questions such as:
* What are the means by which we "know ourselves" - who teaches us about ourselves?
* Is our failures to change really a failure of knowledge? Amongst which group of people?