So, it's pretty clear that she said what she said in order to get MC to look at the girls more seriously and try to evaluate them, in an attempt to deepen their relationships. Since she wants him getting with all of them she can't risk him actually marrying one of them as it's clear MC would be pretty faithful and not go full harem if he only married one, combine this with how there's kinda a relatively limited relationship between MC and the various girls and I think that she was referring to one of two people:
1. Herself. Since even with perfect spying it can be hard to know how someone truly feels and most of these girls have only spent so much time with MC, it raises some questions if they'd have such deep feelings let alone express them in a way that she could absolutely know (meanwhile all the other girls didn't feel that way), but there is one character who she could know their innermost thoughts, herself. She was in the room when she said it, so her statement was true, but it made him look at the other girls which would have him pursue the harem route which is what she wants.
2. Stalker-chan, who was probably in the room but he didn't see. Wouldn't be hard to see her openly stating and writing in a book or something that she wants his babies. The only real question there is if the Love-Note is keeping up to date on the stalker.