Rashi: It is all mixtures of Goyim, like "v'Gam Erev Rav Alah Itam."

Malbim: It is the converts from Amon and Mo'av who became mixed with them. Now, we hold that the nations were mixed up [via Sancheriv, 200 years before Nechemyah], and anyone who separates (any Goy who wants to convert, we assume that) he separates from the majority [of nations, whose converts may marry Yisraelim - Berachos 28]! However, in the days of Nechemyah, there were Amonim of known lineage. Primarily Nebuchadnetzar mixed up the nations (Tosfos Megilah 12), after the Churban, and some of the nations were not mixed yet.