Before taking her advice to pay more attention to the harem, can you name one person from the harem who the emperor even spoke to, let alone invited for dinner? Twice? To say nothing of their "not a date".
You can't, can you? Because FMC was literally the only person in the harem he interacted with at all and the only one he showed anything resembling interest in.
He explicitly stated he wanted her by his side, was literal seconds from murdering her kidnappers, and was more than willing to go full Spanish Inquisition on the other consorts for her.
But sure, there's absolutely no indication to her face that she might just be his favorite.
Let’s say you have a partner.
You live in a dorm of separate genders.
You
went to sleep alone but the next day you
hear from people that your partner left the dorm in the morning, implying he/she spent a night.
You didn’t ask him/her.
Despite being your partner, you start to doubt his/her words of affection.
You know it wasn’t you because you
went to sleep alone.
But the explanation? Your partner sneaked in, slept beside you & left in the morning before you woke up. So it’s actually you.
The bold characters here to emphasise the analogy.
As I’ve said, it’s
understandable. That doesn’t mean it’s right, neither is it saying it is wrong. Your argument puts the narrative into black and white from a reader’s perspective.
They all think he is being secretive. No one knows, neither does she, at least that’s what she thinks. She has not the slightest clue that he spent a night with her. There’s no clause for argument when in her perspective, that’s like a confirmation it was not her.
Yeah, let’s also just ignore all her inner monologues of her thinking he is just doing this to keep it secret.
On the other hand, I only made sure to say it is understandable with this plot point because it’s sensible. But I don’t exactly think this will be the story’s logic because despite all that, she only had inner monologues of the “secret” instead of the part about the sleep. So it may have just been a wasted plot point.