Dave is correct in that the Greek and Latin sources do not mention horse barding for Sarmatian horsemen. Also, the Sarmatian heavy cavalryman depicted on this relief from Tanais does not have any horse barding:
OTOH, this panel from Trajan's column clearly depicts several Sarmatian cavalrymen with full scale horse barding, similar in appearance to that worn by the riders:
So, as usual when dealing with the ancient world, the evidence is rather ambiguous. My guess would be that some of the wealthiest nobles had horses with full scale barding, while those who were less wealthy did not. Rating Sarmatian nobles as having cloth barding in the army list could be a good way of representing a mixture of troops with scale barding and no barding.
What puzzles me is the -2 points for cloth barding. Is that because Sarmatian nobles with cloth barding go from being Shock Cavalry to Massed Cavalry? Or is it possibly because the default was for them to have full or half barding, and so cloth barding would be a step down in protection? Possibly both?
‘Find something you love to do and you’ll never have to work a day in your life.’