We don't actually know whether or not the muramasa can cut adamantium. This is assumed not to be the case by some, because when the tinkerer coated Daken's claws with muramasa magical plasma, he made adamantium sheaths for them to retrieve into and told him to be carefull about breaking them (the claws or the sheaths, I don't quite recall). People believe the sheaths were made of adamantium because a lesser material would be busted by the talons protracting and retracting - if the claws seared daken's flesh like logan's do to come out, he'd be crippled as they prevent regeneration.
However, an equally plausible explanation is that they were made of adamantium to prevent external blows from breaking them, releasing the muramasa into daken's arms wounding him. In this case, if the tinkerer was talking about the claws and not the sheaths being broken, he may have been refering to magical constructs like the armor that broke the sword in the first place (he was given a clearly broken fragment to work with) or he may have been talking out of his ass.