SQL Server is a relational database management system, the entire of SQL Server, not a piece of it.
That page has decided to split it into two broad subsections, but that's just a break into subsections for broad classifications of the components.Go and read the details in there, you'll see how they decided to group the components.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass