I don't think you will find a single person who wrote at the time who made no mistakes.

Nor am I looking for one. I'm extremely uninterested in questions like "was Korzybyski a very clever person?" and much more interested in ones like "does the stuff Korzybyski wrote contain enough insights I currently lack, and little enough distracting wrongness, to be worth my while reading?".

A bit above you mention that "is" can be a treacherous word. Yet you started both of those questions with it.

Yup. There is no valid inference from "X can be treacherous" to "one should completely avoid X". If Korzybyski claimed that one should simply never use "is" (I am doubtful that he did, as it happens[1]) then I disagree with him, which is not the same thing as failing to understand him or failing to have assimilated whatever useful points he may have made.

In the present case, note that "Is that all wrong?" is equivalent to "Does all that have the property of wrongness?" which doesn't (either implicitly or explicitly) contain an "is". (It arguably has the weakness of assuming that everything can be classified as "wrong" or "not wrong", but use of such language is a very useful shorthand; it would be intolerably cumbersome always to have to say things like "Does all that have the property of wrongness to such an extent that I should be regretting having written it?". And "Is there substantially more ..." is equivalent to something like "Does Korzybyski's work contain substantially more useful insight than what I wrote above would suggest?". Again, no "is" contained or implied.

Now, if you think I made an actual error as a result of writing in English rather than in E-Prime, that I'd have avoided if I had more of Korzybyski's ideas in my brain, go ahead and show me; but so far all you've done is to observe that my writing doesn't conform to some rules associated with him (I'm not sure whether they're his or his followers') which I currently see no sufficient reason to endorse.

As far as the charge of being poorly organized and verbose goes, it comes from the book being hard to understand.

It seems somewhat plausible that the causation might go the other way. Are you claiming, specifically, that in fact K's book is neither poorly organized nor verbose, but that because K didn't write it accessibly Gardner misunderstood it and this led him to mischaracterize it as poorly organized and verbose?

As far as the charge of not having influenced anybody goes [...]

I didn't see that charge. I saw a charge of not having contributed anything novel to human knowledge and understanding. Not being very influential (if indeed K wasn't) might be a consequence of that, though it's not hard to find very influential people whose influence didn't mostly come from such contributions.

I wonder what led you to see a "charge of not having influenced anybody" in what I wrote. It seems to me a rather curious error.

[1] My hazy recollection is that Korzybyski and his followers have no particular objection to "is" when used merely to apply an adjective to a noun: "the cat is blue", but that they object to using it in contexts that assert, or seem to assert, the identity of two different things ("that man is a liar", maybe).