At first, she didn't know he is her truly darling, but she had already known someone's name was hiro and she kept learning the "darling" word again and again. Until Hiro remembered everything and asked her about her childhood and relationship between them. So yeah,she recognized him after all kinds of shit lol
She remembers how she met someone in the past who showed her the outside world and didn't treat her like a monster, but she didn't fully remember who her "darling" was. That's why she wasn't totally aware that Hiro was her darling all this time.