Answer

Hello Sonia.
This is the language of the Jewish law:
One interrupts Torah study to hear the reading of the Megillah, and all the more so for the other commandments of the Torah, all of which are set aside in favor of the reading of the Megillah, for there is nothing for which the reading of the Megillah is set aside except for a met mitzvah who has no one to bury him (as needed), so one who encounters him buries him first and afterward reads.
Gloss: And all this applies only when there is time to do both, but if it is impossible to do both, then no Torah-level commandment is overridden in favor of the reading of the Megillah (Ran and Beit Yosef in the name of Tosafot and Rabbi Eliyahu Mizrachi). And the fact that a met mitzvah takes precedence applies specifically when he is able to read it afterward (Rabbi Eliyahu Mizrachi).
The Rema writes that no Torah-level commandment is overridden in favor of the reading of the Megillah, and that includes a met mitzvah.
You are asking based on what he wrote at the end, which seems to imply that if he cannot read afterward, then the reading of the Megillah overrides the met mitzvah. But that is not what he means. His intention is to say that giving precedence to the met mitzvah over the reading is only when he can read afterward. And what if he cannot read afterward? Then he reads the Megillah first and afterward buries the dead person (that is, in any case the burial is not lost). And what if afterward he will not be able to bury the dead person at all? Then of course he gives up the reading of the Megillah, because the reading of the Megillah does not override any Torah-level commandment.