I found it an odd thing to say about an onging investigation.
Our system works on the idea that people are innocent. Prosecutors generally shouldn't make public declarations that a suspect is guilty.
Not that they don't do it, mind you.
It would be odd if a prosecutor, commenting on Mr X, accused of bank fraud, said in public:
"If we had confidence that Mr X did not commit a crime, we would have said so."
I think such a statement would seriously jeopardize the case against Mr X.
The only thing i can think of is that Mueller knows there's no case against Trump, and wanted to get a shot in at Trump.
Because I think as a prosecutor, Mueller would not have wanted to make a possibly prejudicial statement about an ongoing proceeding.
Anyway, we've known all along that charging a sitting President is a job for Congress.
In the case of a President, the system is inapplicable He or she is above any consequences and there is no possibility of what the rest of us would face, a trial. Being above the law is a fact for the Officeholder. Considering that to be the case all the evidence in the world would have gotten the same result. For sake of discussion, there might be an investigation that finds a President is a monster child rapist serial killer of the highest order, yet Mueller could not say that. Instead, he would cite the facts and while he can't accuse but he can say "I say he was innocent", and due to the egregious crimes be they murder or obstruction of an ongoing investigation or any serious matter of national lasting importance, then yes, they ought to say something.
Trump spoke with less credibility than usual just now he wildly accused Mueller, as you have, that this was about getting a "shot" at Trump. Well this is an odd way to go about it as the report outlines obstruction with facts clearly presented. No, Mueller recognized that a criminal was above the law and while he officially could not indict he has a voice and can do his patriotic duty and put Barr and Trump's lies to the test of fire and they both failed.
What should happen is the Trump should be impeached, removed, and then prosecuted at the Federal level. He should have his day in criminal court and not in some remote future. But the Senate doesn't care about crime nor do Republicans in general. Instead they accuse Mueller of revenge and a witch hunt and will not do their duty. That's the really relevant and important thing glossed over, not Mueller yesterday other than him being a man who has a quaint and mocked quality, virtue.