For the most part updating statistics runs in a similar fashion to any other query in the system. The degree of parallelism is set at the system level and SQL Server will use that when running the queries. There's nothing in sp_updatestats that forces MAXDOP 1. You can't force MAXDOP on UPDATE STATISTICS through a hint (although there is an open Connect item requesting that). If you're on Enterprise you can force this through the Resource Governor.