For those still following along, here is part 3 of Lavish's explanation. If you didn't read the first two, that's okay, because he includes a good tldr.
This installment explains how crazy the letter is :
Bottom line: the ISDA Letter seeks to reclassify US Treasuries from risk assets to risk-free assets.
Their argument?
"...to preserve the resilience of the U.S. Treasury markets and the U.S. economy and financial system."
They are basically petitioning to remove them from the calculations, permanently.