Perhaps Texas has extensive discovery in criminal case. But I would very surprised if even very liberal discovery would go that far. The discovery rules generally relate only to facts to be used at trial, exculpatory evidence and tangiible evidence. It would be a most unusual discovery rule that permits investigatory argument to be disclosed. It is generally viewed as irrelevant as to who advocated for or against indictment. If this is a viable rule than logically it would expand to include the for-and-against discussions within the prosecutor's office itself. The indictment itself is the result of the investigatory process and the discussions of dissenters to the indictment are enclosed in the grand jury's vote.
While I am admittedly ignorant of Texas criminal procedure, my professional intuition tells me that the request is more of a press release than a genuine attempt to obtain discoverable exculpatory information.
This is relevant because the defense claim is that this is a politically motivated indictment and that Earle strong-armed the grand juries (more than one). Since one indictment is for something that was not even an offense under Texas law, their is probable cause that political motivation overwhelmed professional judgment.
Second discovery can be pretty broad reaching being designed to develop evidence that can be used at trial.
Either their is evidence that some in the prosecutors office realised that the original indictment was very weak, or there wasn't. Either way, DeLay wins.