Both Herodotus and Xenophon describe million man armies. Herodotus claimed the Persian horde was a million men strong when invading Greece. Similarly, when listening to a lecture the professor said Xenophon listed Artaxerxes' army at "one million men" when facing Kyrus as Kunaxa. Also, from my understanding the Hellenistic zero was used for a fractional part of a number.
So, how did this translation come to be, and what would the original Greek text have stated with a direct translation?
I'm not sure why you think "million" requires a zero, except if you are thinking of writing it in arabic numerals as 1,000,000. But there' no reason that someone has to do that. I don't need a concept of zero to have a concept of ten.
Anyway, to the specifics I presume you are referring to something like Herodotus 7.60:
σύμπαντος δὲ τοῦ στρατοῦ τοῦ πεζοῦ τὸ πλῆθος ἐφάνη ἑβδομήκοντα καὶ ἑκατὸν μυριάδες.
Translation: And altogether the sum of their army on foot was seen to be one-hundred and seventy ten-thousands.
Comments: Herodotus uses 'on foot' here to refer to the land army, not to the infantry. In terms of the numbers, he uses Greek words for 100 (hekaton) and 70 (hēbdomēkonta) as multipliers for 10,000 (muriades). No need for a zero symbol ata all, because Herodotus is not writing out numerals. Even if this were written out as a number in Greek, Greek used letter substitutions for numerals, so again there is no need for a zero symbol.
It's true that Ptolemy uses a symbol for zero in his Syntaxis Mathematica, but this never gained widespread traction for quite some time. Nor is it directly related to your above question.
In short, you don't need a symbol for zero to write really large numbers.
Are you confusing Zero with 0? Zero, the numerical representation of nothing; opposed to 0, the digit representing tens, hundreds, thousands, etc etc?