Papyrus holds off on the dramatic flourish of targeting Lambert's soul, because that's not the reaction he'd been expecting, and thinks back. Lambert was very busy back in Hell, with fighting and not dying. No time to stop and admire the bone constructs, or the effects the various things he did had. Nothing a quick explanation can't fix. But what did he show Lambert at Celebration...?

"Oh! Nyeh heh heh, yes, of course! That was my special attack. For ending fights! It's not much for conversation."

Whether he means that the skull doesn't talk much, or that using that attack interrupts trying to talk during a fight, he doesn't bother to specify.

"That's why, instead, I'm demonstrating the regular kinds of special attack! These are nearly regular bones. You've already sussed out the trick to them: don't move, and they don't hurt. How insightful..."