Ken Starr used the word GUILTY in his report 15 times..that was NEVER used against Donald Trump, not even ONCE, if Mueller wanted to he would have used that in his report even if he couldn’t prove it
To top it off, he says "First, the opinion explicitly permits the investigation of a sitting President because it is important to preserve evidence while memories are fresh and documents are available." And finally, "...the Constitution requires a process other than the criminal justice system to formally accuse a sitting President of wrongdoing."
In other words, "I have damaging evidence for you to dig through" and "It's up to congress to pursue impeachment".
Mueller had no reason to say these things beyond pitching a grenade into the oval office before saying "that's all I have to say" and dropping the mic.