This beraita is not discussing the case of Purim falling on shabbat.
It's talking about villagers who are not able to read megillah on the 14th (on a weekday). Instead, they read on the previous market day (Monday or Thursday) and give out matanot la-evyonim on that day. However, they rejoice on the 14th.
As couple of points are addressed by HaRav Chaim Bleier in his Chukei Chaim series on Parshas Tzav 5779 here
He writes there:
If one began his Purim seudoh during the day and wants to continue it into the night as his Shabbos seudoh, he must stop eating and make Kiddush close to bein hashmoshos before continuing since the chiyuv of Kiddush then takes effect ...
Rav Eliezer Melamed writes here:
[T]here is a custom to combine the Purim meal with the first Shabbat meal on Friday night. Some great Torah authorities follow this custom, while others recommend doing so only be-di’avad. In order to follow this custom, one must pray Minĥa and then begin the meal while it is still Purim. Then, around a half-hour before shki’...