>Caesars were emperors, just of a lower, junior grade to Augustuses

Only after the reform of Diocletian, the so-called tetrarchy. Before the tetrarchy Caesar was reserved for the emperor and occasionally for his to be successor (from the time of Hadrian to the reign of Diocletian). Originally Caesar was used for all male members of the imperial family (Augustus to Hadrian). Since the coin is from before Diocletian, Caesar means that it refers to an emperor.

edit: rephrased the last sentence.