Well, I think he is, up to this point at least. With some effort, I am able to understand him.
I think we might rephrase the sentence in question as follows:
“The Being of that which is, itself something that is, “is” not.”
Which we might, after my example, compare to:
“The running of that which runs, itself something that runs, “is” not.”
Or, according to you:
“The running of that which runs, itself something that runs, “runs” not.”
But “itself something that runs” means: “which [running] is itself something that runs”. So you cannot evade the “is”.
In any case, the “is” between quotation marks is a different use of the verb “to be” than the forms of that verb outside of the quotation marks - as you yourself have taught me. In the former case, it means something in the line of “is equal or identical to”; in the latter case, it means “to have reality or actuality”. Merriam-Webster distinguishes between these two meanings by giving the former the number “1”, the latter the number “2”:
“Being” in the first sense is not comparable to the transitive verb “to run”. We might make it completely unambiguous if we replace “running” by “aspiring”:
“The aspiring of him who aspires is not itself something that aspires.”
This makes sense. The following does not:
“The aspiring of him who aspires does not itself aspire something that aspires.”
This is nonsense because “to aspire” is not a transitive verb.
Now, as for my rephrasing at the beginning of this post:
“The Being of that which is, [which is] itself something that is, “is” not.”
In this way, the “is” between quotation marks means the same thing as the other forms of the verb “to be” (except for the “is” between brackets).
“The existence of that which exists, [which is] itself something existent, does not exist.”
I.e.: that which exists, exists, but its existence does not.