Where does that idea come from? It seems rather dubious, given that evidence of horse archery stretch across the entire Scytho-Sarmatian range, millenia before the advent of the Cimmerians in historical reference; most probably from before the Cimmerians headed west. AFAIK, archaeological finds have also so far pointed to a trend for the western Scytho-Sarmatians to use a higher proportion of self bows, relative to their eastern counterparts, possibly because of a lower abundance of wildlife with enormous horns, such as the argali.
First, evidence of
Chariot Mounted Archery stretches back to at least 1700 BCE in Shang China and 2100 BCE in the area now known as Kazakhstan, but the archeological findings of 'bit wear' in horses mouths in the Botai culture in the Kazakhstan area dating from 3500 BCE are not necessarily consistent with riding rather than driving - especially since the Botai excavations have shown no skeletal evidence of horse domestication but a lot of evidence of horses as hunters' prey. The first unambiguous evidence of Horse Riders rather than Horse Drivers comes from Assyrian depictions of mounted archers and findings of saddles in graves in southern Siberia, and both of those date from the 9th and 8th centuries BCE.
BUT I'm probably being conservative: every new archeological dig seems to push dates back, and the next publication may show findings of evidence like saddles or riding bridles dating back another century or more: all of 'what we know' is subject to change...
Likewise, a primitive Composite Bow - or at least the materials of horn and bone for one, have been found in Sintashta culture graves dating to about 2100 BCE, and the 1700 BCE Shang chariot archers were using composite bows and composite bows were found in Tut's tomb (1324 BCE) BUT the earliest recurved composite bows with stiffened ends for increased power and recovery by the bow so far have been found in Scythian graves and no earlier (again, so far - everything is subject to new archeological discoveries) than the 4th century BCE. IF the Scythians in the west were using Self Bows it went un-noticed by any of their Greek or Persian enemies and neighbors, who all speak of the 'Scythian Bow' as the recurved composite type.
The problem, as I mentioned, is that Every Culture or Group on the Steppes from Mongolia to Rumania adopted a virtually identical horse-riding, mounted archery life and combat style and maintained it virtually without interruption from At Least 7 - 800 BCE to 1500 CE, and the differences among them were, frankly, minor compared to their similarities.
Speaking of Persians, both Roman and Indian sources say the Parthians were a Scythian tribe.
Case in Point for Similarities: people living the 'Scythian Lifestyle' who wandered into Non-Steppe areas with dramatic results include the Parthians, Seljuk and Ottoman Turks, Xiong-Nu, Huns, Jurchens, Mongolians, etc., etc.
