I doubt it’s true. It’s as ridiculous as russian hookers urinating on a bed. If people are as basic as they are claimed to be why do they always cite absurdity. In Hitlers cas he already owns the title of most tyrannical even though Mao and Stalin were considerably worse. When they do that I tend to start the question the more supposedly legitimate aspects of the narratives.
You are right to question when it becomes absurd, I do the same thing.
However, in this case, I don’t think it is as absurd as someone trying to make Hitler look even weirder than he was.
Apparently, “chewing the carpet” is some kind of German idiom, meant to describe behavior in the same way pacing the floor nervously (wearing off the carpet) or biting your nails.
In this case, it is entirely possible a German official he knew was explaining it to him, and the idiom didn’t translate correctly in his mind.
From the way he worded certain things, I get the impression Shirer was a Liberal in the modern sense, but he also tried to be as objective as he could (and said so multiple times in the book, because of the possibility of someone discounting what he wrote because he had a bias...and face it, who wouldn’t?)
I tend to think someone said “He fell to the floor and began chewing the carpet” (in the same way we might say “he began to chew his fingernails down to the quick” to describe someone showing overt signs of inward stress) and he, not being a native German, might not have recongnized the idiom.
The rest of the book is good enough that I give him the benefit of the doubt on that one, and while I think in 1960 one of the Germans he probably had reviewing the book for edits may have warned him, in 1960, only 15 years after the fact, maybe nobody in Germany wanted to be seen sticking up for Hitler and saying he wasn’t really that crazy!