The hearing is a tantrum over the DOJ's sentencing recommendation in the Stone case. While Dems are quibbling whether the Feds should have pointed out that a "threat" Stone had made was not serious, they are ignoring the simple fact that the judge was completely familiar with the matter and had the discretion to give the augmented sentence the Dems wanted. The judge, not the DOJ, decided what Stone's sentence would be.
The Dems' witness was a disappointed member of the Mueller team. The Rs' "witness" is the judge who decided that the DOJ's reasoning was correct.