> the Seleucid Empire where it was named after the ruling dynasty?

​

Herein lies your error. "Seleucid Empire" is a modern term, not one that the Greeks typically used. The Seleucid kings certainly at no point ever would have used such terms to describe their own kingdom, and following the pattern of Antigonus and the Ptolemies all Hellenistic monarchs regularly referred to themselves e.g. in coinage and other official texts simply as "βασιλεύς," king. King over what? King, typically, over no particularly definable area. The Hellenistic monarchs had, typically, little interest in defining the extent of their own realms, since their royal titulature had developed out of the multiple claims to Alexander's kingship. To the Hellenistic concept of monarchy, kingship had no limit, nor was it fixed to any particular state entity besides the king himself.

​

Greeks and other observers referred typically to Hellenistic kingships either by the names of their kings, or rather less frequently by the names of the people over which they ruled. Thus Syll 434-435, the decree of alliance with the Spartans and Ptolemy in preparation for the Chremonidean War in 267, makes an alliance with the Spartans and their allies (and any of the Greeks who wish to join the alliance) but does not mention the Egyptians, only that an alliance is also to be concluded by the Athenians with Ptolemy, personally. This is pretty standard.

​

The Indo-Greek kings follow the same standard. Demetrius minted coins that simply said "βασιλέως Δημητρίου," of king Demetrius, this being the universal way of minting royal coins in the Greek east. King of what? King of nothing, just "king." When our few sources refer to Demetrius and his kingdom, they simply refer to Demetrius. There is only one place where Demetrius is not simply called Demetrius or king Demetrius, and that's at Strabo 11.11.1, where he is called "Δημήτριος ὁ Εὐθυδήμου υἱὸς τοῦ Βακτρίων βασιλέως," Demetrius son of Euthydemus the king of the Bactrians. The "king of the Bactrians" bit actually refers to his father, since Demetrius' name is in the nominative, but the point still stands: like other Hellenistic kings, the Bactrians and Indo-Greeks were referred to, if precision in geography was needed, by the peoples over which they ruled, not by any concept of a greater kingship or empire. Indeed, the Hellenistic monarchs might rule over multiple kingdoms, but not a united imperial polity--Strabo in the same passage refers to the Greek conquest of "τήν τε Σαραόστου καλουμένην καὶ τὴν Σιγέρδιδος βασιλείαν," the kingdom called Saraostan and the one called Sigerdan. But he does not say that the Greeks were kings "of Saraostus and Sigerdis."