Whatever it was, Ignazio Silone didn't like that the Reverend had said it. I can't find out who the Reverend was or what Silone disliked in his 1950 speech with my meager Google-fu.
So the fatal flaw with this is that "feeling threatened" is subjective. A fragile person with zero resilience will feel threatened by you, even if everything you do is harmless.
This makes it unusable as a scale for what is acceptable speech and what is not.
On the other hand, "Everyone should go kill that person" is pretty much universally seen as unacceptable. So you can't get out of the ambiguity by declaring all speech defensible.
The law lives with ambiguities all the time, and has various means of resolving them. They are imperfect, but absolute clarity is not something you can achieve from the law, and it is sometimes counterproductive to try.
regardless of your views on the titular quote, that discussion isn't really germane to the article which is simply investigating the origin of the quote.
so if I have any power as a random commentator to shape this discussion, perhaps we could talk about how the distortion occurred because of the placement/presence of quotation marks
and how similar popular misconceptions might have formed through punctuation.