Which historians have claimed a number that high? I do remember reading that Judaism was much more active in seeking converts in ancient times than in Medieval or recent history, so it would make sense that many modern Jews are descended from converts.
The source usually cited is Syrian Bishop Bar-Hebraeus, who was citing now-lost source material from a census in the time of Claudius.
This figure is usually dismissed as impossible, but might refer to semi-converts who gave up Roman beliefs for Jewish ones but did not become official converts to Judaism.
This new study may be yet another interpretation for an unusually large number of Jews in Roman times.
The population of the Hellenistic eastern Med including Hellene-occupied Israel and Egypt (Alexandria had a very large Jewish minority), Jews lived in every ancient city in the Mediterranean and not just the ports), including Rome, so Roman conquest brought a lot of Jews under Roman dominion. The Jewish population estimate most will accept is 10 percent, but at its peak it may have been as high as 15 percent, IOW, wow.
The catamite-lovin’ homo Emperor Hadrian despised the Jews for things like what he considered body defacement (circumcision, in this case), but Roman persecution of the Jews probably hit its peak under Diocletian, with the Flavian dynasty right behind. As the Jews kept their Sabbath holy (no one else actually *had* a Sabbath), they were ineligible for service in the Roman army, though they sold the many essentials to keep it running. They had only one God, and the Romans had many (and kept making more by Senate decree), and it wasn’t an idol but an invisible presence; that freaked out the Romans and kinda pissed them off.