I'm not sure if I like where this is going....best to just ignore this statement.
If anything, the early Slavs probably did have a connection with the settled agricultural classes of the Scytho-Sarmatians and there is tons of linguistic and circumstantial evidence to support this statement. It still doesn't mean that the Slavs should have awesome cavalry. There is the theory that the Antes were a Slavicized Alanic tribe with a Sarmatian ruling class but even so, Maurikios stated that the Slavs were mostly forest dwellers - spearmen, skirmishers, and ambushers and lacked significant cavalry. As late as the late 1200s the medieval Rus principalities still struggled to field significant cavalry of their own without hiring runaway Polovtsi.