If I recall, I believe there's a tendency in the Church Fathers to refer to "heathen" or "pagan" (non-Christian) nations as τὰ ἔθνη (in a non-ethnic sense). But here it seems that Eusebius is clearly using ἔθνη as a term for "outsiders" from a Jewish (ethnic) perspective. And, presumably, he would self-identify in these terms as a Gentile (ἐθνικός).
Is this assessment correct? Or might I be missing some nuance?
Eusebius, Ecclesiastical History, IV. vi.3-4 (c. 313 AD)
Thus when the city (Jerusalem) came to be bereft of the nation of the Jews,
οὕτω δὴ τῆς πόλεως (Ἱερουσαλήμ) εἰς ἐρημίαν τοῦ Ἰουδαίων ἔθνους
and its ancient inhabitants had completely perished,
παντελῆ τε φθορὰν τῶν πάλαι οἰκητόρων,
it was colonized by foreigners,
ἐλθούσης ἐξ ἀλλοφύλου τε γένους συνοικισθείσης,
and the Roman city which afterwards arose changed its name,
ἡ μετέπειτα συστᾶσα Ῥωμαϊκὴ πόλις τὴν ἐπωνυμίαν ἀμείψασα,
and in honor of the reigning emperor Aelius Hadrian was called Aelia.
εἰς τὴν τοῦ κρατοῦντος Αἰλίου Ἁδριανοῦ τιμὴν Αἰλία προσαγορεύεται.
The church, too, in it was composed of Gentiles,
καὶ δὴ τῆς αὐτόθι ἐκκλησίας ἐξ ἐθνῶν συγκροτηθείσης,
and after the Jewish bishops the first who was appointed to minister to those there was Marcus.
πρῶτος μετὰ τοὺς ἐκ περιτομῆς ἐπισκόπους τὴν τῶν ἐκεῖσε λειτουργίαν ἐγχειρίζεται Μάρκος.