What Heinz Linge is observing is likely Goebbel's propaganda machine at work, in particular the 'cult of personality' which essentially painted Hitler as a genius and Reich leader. Propaganda is extremely effective - if you heard from every one of your news sources that Hitler was a genius, you would likely beleive it too.

What Hitler was very charismatic, and an excellent orator. This can be seen in the videos of his speeches. However, while it is possible he had a 'genius' IQ (or however else you want to measure intelligence), there is certainly lots of evidence that Hitler wasn't all Goebbel's cracked him up to be.

In fact, historians Hans Mommsen, Ian Kershaw, Jeremy Noakes, and Martin Broszat have all considered him a 'weak dictator'. They have noted how his personality was generally lazy and erratic, and how he was easily bored and uninterested with detail (especially economics). For example, he would often set ministers with overlapping duties, forcing them to compete for influence, and causing a "permanent confusion over planning, especially in war years".

Finally, often times during the war he would dictate battle strategy in place of his war generals, which would lead to serious battle mistakes that a seasoned veteren wouldn't make. Although I don't know much in this area (correct me if I'm wrong), but I beleive that he often implimented policies of "fight or die" (ie. no surrender) which lead to inflated death counts.

In other words, there were a lot of instances where we can see Hitler was not the 'genius' Goebbels made him out to be. You say these people who beleived him a genius 'were not dumb', but again I think you underestimate the effectiveness of propaganda; all dictators have used propaganda, which is still affecting us today.

Source: My main source is, "Authoritarian and Single Party States" by Alan Todd and Saller Waller. If you want more info, just ask!