I don’t really expect Smash to have such a high pedigree of story where spoilers are a gebeuine concern. I predict Kirby will get the ball rolling, start saving other characters somehow, and once everyone is freed, they will once again take on Galeem and win.
Now think about this. The day before an open-world game comes out, say Breath of the Wild, this video comes out showing the entirety of Hyrule. I mean, literally all of it. Every single area, town, dungeon, shrine, interiors, everything. Even the final dungeon. It also plays the soundtrack to each of these locations as it goes by, and shows the characters who are in said places, including the bosses (and final boss), but it doesn't give any story spoilers away.
Would that be a spoiler?