It's probably cause they read the novel in Kakao or any raw site they gotten ahold of and shared their understanding(or MTL) there. I'm just saying the novel for this exists and afaik still ongoing. I used to have the Kakao app and bookmarked it so I am sure the novel exists but its in raw form which is Korean and I haven't seen any fanmade english translation for the novel. Direct link for the novel is already on that forum too so you can check it if you want.
Peacock kind of shot himself in the foot with that premise. Melissa can condemn him for insincerity and false flattery if whatever he picks doesn't derive its value from its relationship to Yuri. Surely the most expensive dress in the land is less valuable than a dress worn by her, so the logic goes; if he doesn't consider things that belong to Yuri to be 'the most priceless and precious in the world' then his love must not be true and he's a materialistic, two-faced cad. And if he does buy something that's valuable because of Yuri, well, he's procured something that belonged to her from a third party without her consent, which makes him a different kind of cad.