Is Muhammadism a pagan idolatrous religion like those east of the Euphrates?
No. The context distinguishes religions east of the Euphrates as predominantly pagan and idolatrous, while west of the Euphrates it lists Judaism, Muhammadism, and Christianity. Muhammadism is explicitly described as not akin to the pagan idolatrous religions of the east.
Les explains that the Euphrates is the great boundary between east and west. East of it, religions are predominantly pagan and idolatrous, with roots at the Tower of Babel on the Euphrates, where all pagan mythological religions began. West of it, he lists Judaism, Muhammadism, and Christianity, and he states that Muhammadism is certainly not akin to the pagan idolatrous religions of the east.