In book "The Wars Of The Jews" written by Flavius Josephus. We may find this record, about Simon Thassi, in Book I, first chapter and second passage.
He was afterward an auxiliary to Antiochus, against Trypho, whom he besieged in Dora, before he went on his expedition against the Medes;
But in other, more numerous, resources we find that there was a war between Parthia and Seleucid Empire.
Why did Flavius Josephus write this? Was this a mistake? Did he consider them one the same nation?
In all surviving literary works written in Ancient Greek, "Medes" (Mêdoi) is used as a synonym for "Persians". Frustratingly, this is true even in texts (like Herodotos' Histories) that treat Medes and Persians as distinct peoples with their own histories and customs. The distinction tends to be made only where it is necessary to tell the two groups apart within a particular context; elsewhere, authors freely use Mêdoi as well as Persai when they are clearly referring to Persians, and the Greek word for choosing the Persian side or adopting Persian habits is mêdizein, "to medize".
In this particular passage, Josephus extends the synonym to cover the Parthians as well. After all, they were a dynasty ruling in Persia, and were therefore Persians/Medes.