R’ Yehonason Eibshutz explains that what the Jews began doing was a continuation of what they started doing at Sinai. For this reason, although there is a prohibition (Devarim 13:1) to add mitzvos to the Torah, Mordechai had the authority to add the mitzvos of Purim in this case. The nation achieved atonement because they listened to Mordechai, and the sincerity of their re-acceptance of Torah allowed for the Torah to be new.
The Sfas Emes and Maamar Mordechai both note that in this verse, too, the Jews did before they learned, in the same fashion as their saying (Shemos 24:7) “naaseh v’nishmah” (“we will do and we will listen”) when they first accepted the Torah.
According to R’ Dovid Feinstein, the word for “ready” as written (atudim) with a vuv implies permanence, in a state of remaining. In other words, the Jews should remain ready for future events. He quotes the Talmud (Shabbos 88a) about the Jews being miraculously coerced by H-Shem into accepting the Torah at Sinai under a threat of annihilation. In contrast, the Jews re-accepted the Torah at the end of Megillas Esther (Esther 9:27) under no such threatening pressure, and under not such obvious miracles.
Ginzei HaMelech writes that this could also be an allusion to the continuing future battle of the Jewish people against Amalek. He quotes the words of the Rambam (Mishneh Torah, Hilchos Megillah 2:18) that all of the works of TaNaCh will no longer be needed once Moshiach comes. The exception to this is Megillas Esther. The Ginzei HaMelech explains that the war against Amalek mentioned in the Purim story will still be relevant after Moshiach. It is a day for which the Jews should continually be prepared.