zwei Millionen Menschen
Is the "Millionen" measurement a noun in the above phrase?
zwei Millionen Menschen
Is the "Millionen" measurement a noun in the above phrase?
Yes. It is a noun in the above phrase.
Proof lies in the dictionaries: https://www.dwds.de/wb/Million and https://www.duden.de/rechtschreibung/Million
Yes. The pattern is
<number> <unit> <measured item(s)>
Examples are
80 Gramm Schinken
Zwei Liter Milch
Vier Tassen Kaffee
Drei Teelöffel Salz
Zwei Stück Zucker
Sechs Dutzend Eier
Zwei Millionen Euro
In the last examples you see that the unit is a number-nouns, but they still are used as units