If people can be said to have a peremptory manner or to speak peremptorily, then I guess you can apply the adjective directly to the person. However, I do tend to agree with Bill that the usage is a little odd. I don't think that it implies any real authority, merely assumed authority. Of course, a person acting peremptorily may well have the authority to back up his or her manner, but I don't think it's a requirement. Think of a two-year-old digging his heels in. "No! Won't!" Peremptory, yes, authorative, no way.