[Avodah] Who wrote Megillas Esther?

[Marty Bluke]

R' Wolpoe wrote:
<Contrary to seder olam, modern historians have identified
achashveirosh with Xerxes the great. This means that the 2nd beis
hamikdash was standing already.

There is only 1 small problem. The same Chazal who identified
Mordechai as a member of the Anshei Knesses Hagedola state in many
places (not just Seder Olam) that the story of Purim happened before
the second Beis Hamikdash was built.

Here is 1 example:
The Gemara in Megilla (11b) comments  on the pasuk "bishnas shalosh
lmalcho asa mishteh" that Achashverosh only made the party after he
calculated that more then 70 years of Galus had passed and therefore
the Jewish people would not be redeemed and the Beis Hamikdash would
not be rebuilt. For this reason at the party he took out the utensils
from the Beis Hamikdash.

I don't see how we can accept the opinion of modern historians when
everything we have from Chazal (various Gemaras in Megilla, medrashim,
seder Olam) and the Rishonim (see for example the first Rashi on the
Megilla)  all say that the story of Purim happened before the second
Beis Hamikdash was rebuilt. What happened to following the mesora?