I didn't understand this also, so you're not alone in this lmao
(I haven't read the novels so I might be wrong but anyway--)
From my understanding it may be him questioning Adele's truth by proving that she indeed doesn't have those on her body which of course by means of showing her naked body... However if she does have a beauty mark or anything that contradicts Adele's "bff"s statement then there's a possibility that those rumors, her bff's been spreading or telling everyone, could be false and exaggerated. Note that the rumor isn't just about adele born as a commoner, that btch might have been spouting nonsense like she's a wh0re and that she's selling her body.
Basically:
No beauty marks = rumors are true
Presence of beauty marks = rumors are false, Adele's bff is a liar.
But with how things are going, she may refuse and actually confesses the truth. lol. I wonder what karma will that two faced bitch get, tho..
"You should not have a single hair or beauty mark on your body"?
Idk if I'm slow or it's out of context? Can anybody explain?