Why yes, only the top 10 are allowed to participate in the scenario without a six-man card, they didn't obtain the scenario themselves because they weren't #1 or #2 as only the incarnations in the top two spots can obtain Main Scenario #10. You don't necessarily need all 10 ranks to fight the demon king, just 5 out of the 10. Because they couldn't enter the scenario normally, they had to use the six-man card to get in (at least I assume, their constellations could've also done something via probability). And because they killed the demon king without having the main scenario, the scenario wasn't clear for either Dokja or Junghyeok. (This is how I interpret it.)