Question

Honorable Rabbi, hello,
in Berakhot 2b the Talmud brings opinions regarding the time for reciting Shema: Rabbi Eliezer says, “the day becomes holy on Sabbath evenings,” and Rabbi Hanina says, “when the poor man enters to eat his bread with salt.” The Talmud explains that the time of the poor man is certainly later than that of the priest, and Rashba explains that Rabbi Eliezer’s time is the beginning of sunset, and afterward Rabbi Meir, and afterward Rabbi Yehuda. Therefore it is impossible to say that Rabbi Hanina is before Rabbi Eliezer, since according to everyone it has to be night.
And this is what I found difficult: seemingly from the beginning of sunset there are several stages, and it could be after Rabbi Eliezer’s time during twilight or a bit after the sanctification of the day. But how do we know it is after the priest’s time?
Thank you.
P.S. Sorry if I went on too long.