A little of both. It was decided by Mitch. But he acted knowing that that from a sheer parliamentary point of view he could- in theory- be overruled by various procedures in theory- except he knew the votes were not there to do that.
If the majority of republicans on the Senate Judiciary Committee disagreed with Mitch they could have still voted Garland's nomination out on to the floor or to hold hearings anyway, but they all went on record as supporting Mitch so there was no play there. And they went on record before the nomination was even referred to them. And in theory, the full Senate can vote something out on to the floor even if a committee votes to kill it but the dems did not have the votes to do that either. But then again, how do you get a nomination even referred to a committee if the speaker won't allow it. It can be done but requires lots and lots of votes by the full senate. Easier said than done.