It's on topic because it's not off-topic. Feel free to downvote it if you think it's a bad question, but "bad" and "off-topic" are very different things on stack exchange sites.
The list of things that are on-topic for this site, as described in the Help page, includes:
- Plot, character, or setting explanations
- Behind-the-scenes and fandom information
The question you specified easily falls into one or both of those categories.
Of course, not all such questions are automatically on topic. The kind of questions that are about sci-fi, but still off-topic for this site, include questions that do not have a single, objectively correct answer, such as:
- Personalized recommentations
- Recommendation/Categorization Questions
- List Questions
The question you linked to is not any of those things.
Thus, it falls into one of the on-topic categories, and doesn't meet any of off-topic exceptions, so it's on-topic.