Not catching that is quite alright, because Papyrus doesn't particularly mean to bash Strange, not even to Strange's face. It's not like all this was his fault. Or even the Ringmaster's, for not giving them radios. The only fair bashing would be to Lilith herself, if he ever meets her, and boy he's not in a hurry for that.

Besides, Strange did tell them about Lilith.

"Would you like to?" He wonders, shifting his grip so he can settle her onto her feet - gripping just enough that she won't fall even if her legs give out. "I didn't see any big injuries... but, pay attention to what your body tells you. It's okay if you try and decide not to! You've been through a lot."