More likely, Mueller is just trying to gin up some charges he can use to get somebody like Manafort to lie about what happened so he can use it against others. That's a tactic used by dirty prosecutors who lack real evidence. Many times the jailed "witness" recounts their accusations after the trial that convicted an innocent person.
And when they recant the prosecutor uses the old “so if you lied then how do we know you’re not lying now” trick.