Excellent point. And the story makes reference to a letter that Grassley and Graham have sent to Rosenstein and Wray, not Sessions. It appears that the letter asks for a review of the evidence, and the appointment of a special prosecutor if the evidence warrants. I'm not sure what evidence that Graham and Grassley have seen, but it sounds explosive.
And it therefore infers, if not asserts that Mueller has been too busy trying to take down the Trump administration, to even take the appropriate time to validate whether his witnesses and their evidence was even sound to begin with. And therefore it may even require a new special prosecutor to evaluate the soundness of the the current one, if not his removal completely. If nothing else, Mueller's free wheeling days are reaching an end, as he's facing a true legal attack on his rear flank.