I’m guessing *I haven’t read raws or anything so this is all just conjecture* that there’s a certain reason why she can’t marry 15 atm. Because of this, she’s chosen that random beggar in hopes that she can use him and then swap the two out later. When she insists that 15 isn’t the Yi guy, that’s not necessarily a bad thing... it means she sees him as him so my guess is that this is all a whole elaborate setup just to get w that man.
And w that said
She’s pretty fucking stupid if that’s true for not letting 15 know so he has to go through a shit ton of heartbreak. Hopefully that isn’t the actual plot lol