Little do they know, Pinkie had planned for them to figure that out all along! Just probably not in that specific way. Transcript:
Twilight Sparkle: Well, that was easy. Two turns and Cerberus has been completely pacified! PM: Yep. You did it. Applejack: Hmm… Fluttershy: I don’t think Cerberus really wanted to hurt anypony. This could have gone very differently otherwise… Twilight Sparkle: As fun as facing down a mythical beast was, I’m still wondering why Cerberus is in Ponyville. In Equestria, really. Applejack: Ah’ve got a better question. Why has our happy-go-lucky PM suddenly developed a poker face? PM: Mmph. What Are You Talking About. (beat) Twilight Sparkle: Tartarus. Cerberus being here means Tartarus is unguarded! PM: PPFFFFF- Maybe.