There really isn't any reason for a mention of Citron in that letter.
This was an inquiry by someone (who, I believe, was later identified in one of the iHub messages on the board) looking for records about an investigation of CytoDyn.
The response just stated they can not share anything, even if they have it.
The SEC would not mention any other party that it may be investigating as it was not the topic of inquiry, it is counter to keeping the investigation confidential to maximize effectiveness and it would jeopardize the outcomes of such investigation.
We'll all learn the outcomes when, and if, the SEC and DOJ publicize their findings and actions. Until then its a guessing game.