These additions are recognized as additions. They do not fit in to the general text. The gemara in Megilla does analyze if megilas esther is considered holy or not, ie does it contain Ruach hakodesh. But the Purim story is accepted by all as historical. The Septuagint has other additions, and Josephus quotes it, but Purim was an established historical fact. Taanis Esther is the strange one, not Purim. Here is the relevant Josephus quote (antiquiites XI) , showing that Purim was celebrated by all long before rabbinic Judaism became the sole branch to survive the Destruction:
In like manner the Jews that were in Shushan gathered themselves together, and feasted on the fourteenth day, and that which followed it; whence it is that even now all the Jews that are in the habitable earth keep these days festival, and send portions to one another. Mordecai also wrote to the Jews that lived in the kingdom of Artaxerxes to observe these days, and celebrate them as festivals, and to deliver them down to posterity, that this festival might continue for all time to come, and that it might never be buried in oblivion; for since they were about to be destroyed on these days by Haman, they would do a right thing, upon escaping the danger in them, and on them inflicting punishment on their enemies, to observe those days, and give thanks to God on them; for which cause the Jews still keep the forementioned days, and call them days of Phurim [or Purim.] (21) And Mordecai became a great and illustrious person with the king, and assisted him in the government of the people. He also lived with the queen; so that the affairs of the Jews were, by their means, better than they could ever have hoped for. And this was the state of the Jews under the reign of Artaxerxes.