Not at all. When it comes to numbers Herodotus is quite well known for giving highly exaggerated accounts--armies are not the only victim, he also describes walls as being impossibly tall. Ancient authors in general tend to give exaggerated figures for enemy forces in battle. It's quite understandable, given the "fog of war" effect--as late as WWII Allied soldiers were given completely ridiculous estimates of German forces, sometimes reporting entire divisions that didn't exist and regularly increasing the size of (often quite understrength) enemy formations by quite a bit. There's also the fact that in Greek words for enormous numbers don't necessarily have a definite quantity to them. Μυρίος, the word for 10,000, is the largest number that Greek can express with a single word. But μυρίος almost never actually means 10,000, it means "uncountable, myriad." In the same way that we can say that there were 10 billion, or a hundred million, or even something as small as "over 9000" to mean "a crapton," Greek can do that as well, and Herodotus does it an awful lot