The Rishonim discuss the validity of this reading. How can a townsperson, who is obligated on the fourteenth, read on behalf of the villagers at an earlier date? The Yerushalmi rules that a townsperson, who is obligated on the fourteenth, cannot read the Megillah for people residing in a walled city on the fifteenth.
Turei even and Reb Akiva Eiger explain that Rashi agrees that the townsperson cannot discharge the obligation for the villager. All Rashi means is that since the people in the village are not capable of reading the Megillah; the townsperson reads the Megillah out loud and the villagers repeat after him word by word. The Baruch Ta’am explains further that all the villagers have a Megillah opened before them for otherwise; they would not be fulfilling their obligation since they would be reading the Megillah by heart.
The Ritva disagrees and maintains that a townsperson can discharge the obligation for the villager by reading the Megillah for him. He is not considered someone who is not obligated in the mitzva since he will be subject to this obligation on the fourteenth. It is not comparable to the Yerushalmi’s case. A townsperson and a resident from a walled city have two distinct obligations and two different times. A townsperson must read the Megillah on the fourteenth and not on the fifteenth, and a resident from a walled city must read on the fifteenth and not on the fourteenth. A villager, on the other hand, has the identical obligation as the townsperson and that is to read the Megillah on the fourteenth. There was a leniency established for a villager that he can also read before.
The Rosh in Yevamos and the Ran here learn a different p’shat all together. They state that a villager would be the one reading for the other villagers. The reason they didn’t read for each other on the fourteenth is not because they weren’t proficient; rather it was because it was not customary for them to gather together in the villages on the days of the gathering.