I thought it was rather obvious. Mueller is a prior Marine officer. Officers never truly retire and can be recalled to command. (What is special about Marines on top of that?).
Mueller has a team of Military Intelligence/lawyers assisting in the filing of sealed proceedings. The expansion to "irrelevant" topics is the "bigger than you know".
Mueller, himself, is a perfect Satsuki Kiriyuin for the purpose. The "Deep State" thought they had him and RR for a number of reasons. Keep in mind, Obama's presidential validity was a tool of (((their))) design. The whole political system could be thrown into disarray by Obama pulling a "I never really could have been President and all of my appointments were invalid…" When certain appointees begin to cause trouble.
They thought they had a pretty good series of traps laid out that would destroy any attempt to undo them by those coerced/fooled into serving their interests.
So, they were more than willing to let Mueller be the one to be coerced into playing along with the Trump Collusion story…. And gave him the rope to hang them with when his commission to special council was given wildly exaggerated powers (probably by /our guys/).
"We" know how they have stacked the deck, and because of that, we know exactly when and how to turn each of their cards back onto them. Even when they decide to try and change things up by not playing on their turn, we can still make advances that push them out of the game until they have no choice but to play from their stacked deck and have it turned back on them.
They think they are blue, but have horrible library management synergy.