@Efkius technically the "side story" the author mentions has already begun since the seals have appeared. I guess it was a nice way for him to swim through the plot while making sense.
However now that he's done such world building around the seals and the empire and other countries, to just shove it aside and call it "the end.... the rest is a snack !" leaves me with a bitter taste.
But I get it that after 9 years, he may want to leave this and continue something else/start another story. I just wish he'd have gone through the seals / empire arc and wrap it up at this, that's a tad' too early imo.