Larry,
An arranger keyboard is one with styles and sounds. In actuall musical terms, you are correct.
An Arranger is someone who takes a song and creates individual parts for several musicians to play in a band. The term arranger in keyboards is having the ability to play this keyboard with 1 or more fingers. What you are playing is a simple or complex arrangement with very few fingers.
A workstation keyboard is one with a multitrack sequencer and you create an arrangement after you've recorded these tracks.
The SD5 has all these styles which you play with 1 or more fingers and thus is defined as an arranger keyboard.