Don McGahn is a tool. He's just a tool with slightly more interest in formality than the rest of the swamp.
The rest of it is inconsequential.
Although Mr. Cox had been fired, his staff — duly appointed federal prosecutors — had not. The grand jury, as an arm of the judicial branch, could not be fired by the president. Indeed, Judge John Sirica of the United States District Court immediately summoned the grand juries (there were two) to his courtroom and exhorted them to continue to pursue their investigations and assured them that they could rely on the court to safeguard their rights and preserve the integrity of their proceedings.
In the face of Congress’s inability to obtain evidence that the grand jury well knew incriminated the president, we prepared the grand jury report to Judge Sirica and requested that he use his plenary authority to transmit that evidence to the House Judiciary Committee, which had already commenced a proceeding to consider Mr. Nixon’s impeachment. It was carefully written to avoid any interpretations or conclusions about what the evidence showed or what action the committee should take. The report contained a series of spare factual statements annotated with citations to relevant transcripts of tapes and grand jury testimony. Copies of those tapes and transcripts were included as attachments.
The grand jury has been empaneled since August 2017. He could die in a fire right now and things keep a rollin' along.