Didn’t we already have an entire episode specifically devoted to showing how big of a threat the Saiyans pose?
Look, I’d be the bigger fool for trying to poke holes in DBZ’s pacing. I’d have to willfully ignore how this filler’s been ridiculed enough times over two decades to be a cliche. All the same, it’s undeniably amusing to realize this episode was most likely pitched and sold with the simple narrative goal of having the Z Fighters realize what the audience has already known for several episodes.
It’s the sort of humbly-scoped, tempest-in-teapot plot that really only exists because an adventure franchise is in a particularly comfortable position. It honestly makes me think of an “inventory story” in a superhero comic. Publishers get at that comfortable position with a title, they put out all sorts of annuals, anthologies, holiday specials, ad infinitum, and these books are filled with little one-off stories where the heroes deal with challenges that are much more specific and of much lower stakes than the larger-than-life threats they handle in the main books.
I suppose that might just be “filler” by another name. Most publishers use them as low-risk projects to try new talent out on, and I wonder if that might’ve just been what Toei here.
They’ve got yet another open programming slot because they’re waiting for new manga material, and they’ve already let more senior staffers take their crack at previous filler episodes, so now they’re giving some assistant a shot. That assistant’s tasked to come up with a moderately-exciting episode that puts the characters through a conflict that seems like it has an arc, but don’t actually change anything.
So, while this assistant's coloring within the lines, he works out a yarn where the Z-Fighters enter some magic illusion room and have a big fight with villains who aren’t really there, so there are literally no stakes. They come out with a greater appreciation for how much of a threat the Saiyans are and resolve to train harder - - which isn’t a change so much as a reaffirmation of what they were already doing.
Textbook filler, to be sure. And it's not as good as some previous filler episodes, but I don't know... I think I actually liked the quaintness of it.