the DTCC appears to me to be on the defendant list..
No, they aren't. That entry is not a named person or a legal entity. Not surprised you don't get it.
Here's an analogy. You hire a plumber to unclog some pipes. After the work is completed, he tries several times to collect payment w/ no success. So, he decides to file a lawsuit and names your toilet as the defendant rather than you. Assuming the judge is stupid and doesn't throw out the case, you have no legal obligation as the owner to defend or pay any judgment made against the toilet.
Same applies there. Go ahead and try to hold their computer system accountable.