He doesn't object to either of their realizations. "Well! That stands to reason! I haven't announced it around, there's no reason you would have heard about it."

Seen the signs, maybe, but not especially. He's not the administrative, authoritative sort, outside of (trying to) boss his brother around. Being quick to encourage at the cinderspirits, tell them what to do, isn't the same. The biggest hint is the shape of the crystalline growths, which don't necesssarily mean anything.

"We're more..." Sistina begins, hesitates, and continues. "'King' Papyrus is more... an interim king. Once we're done here, the old king will be back... and, and."

Papyrus takes up at her second hesitation. "It's not a fun job. Not when they're all so worried, and scared..."