It's in passing but in that same book Montfort has a fictional interlocutor say "What on earth are you wearing it on your belt for, fanatic that you are? Why don’t you go the whole way and wear it around your neck like the Spaniards?" which presumably means that that was a popular thing to do, at least in Spain, when the book was written.
He also says that in 1679 Pope Innocent XI granted to those who openly wear the rosary out of devotion and to set a good example: a hundred days' indulgence
The principles to condemn the practice are shaky, I agree with TomGubbins, many people wear rosary bracelets, how is that substantially different? It seems like justifying a cultural hangup after the fact. I know that here in Spain and in some other countries some Catholics wear it piously just as one would wear a crucifix or medal.