Arnold Schönberg was born in 1874 in Austria and moved to the USA in 1933 after the Nazis took power in Germany where he was working. He changed the spelling of his name to "Schoenberg". English was a second language for him.
This panegyric was probably written in 1944. Schoenberg's widow Gertrud found it among his papers after he died in 1951 and sent it to Ives. Online sources all give the same version of the text in English and differing versions in German, so I think it is safe to say either the original was written in English, although there is a very slight possibility that Gertrud Schoenberg translated it from German.
Since English was not his first language the use of the word "negligence" is very probably a mistake and Schoenberg meant to say "neglect" (although "disdain" might have been an even better word). He might have been thinking of the German word "Vernachlässigung" which can mean either "neglect", "negligence" or "disdain" depending on context.
Note there is another mistake in the original quote: the word "of" is missing in the second line. Editors have added the word later.
Changing "negligence" to "neglect" makes the meaning of the line clear. Ives regards the neglect of his works with contempt.