FWIW I think the OP has done all she can for her daughter. Her daughter has opened a door, which goes with the idea of being child-led in discussions of this nature. OP has left it open and is willing to discuss the subject further as and when her daughter chooses too. She has given no judgement and is supportive - whether bisexuality or any other orientation is intrinsic or considered a choice combined with an identity is part of further discussion based on words her daughter used, and down to them to navigate at an appropriate time.
Children's natural sexual awareness develops over time and puberty approaching ramps it up, and it's all wrapped up with lots of other feelings about attraction which isn't necessarily sexual - unpicking this is something that comes with age and experience.
One thing I will say is that parents are caught in a bit of a double bind here - on the one hand there is a far greater awareness of safeguarding and the need to protect children from abuse, obviously this is a very good thing. Part of safeguarding is to recognise hyper sexualisation which may be caused by abuse going on. At the same time, issues around sexuality need to be discussed safely with children when they bring them up. It must be done age appropriately, and the message that who you love is your business is fine. Discussing the nuts and bolts of what bisexuality or homosexuality or heterosexuality means in practical terms is a handle with kid gloves scenario IMHO.
As a PP said, it is very difficult when there is a jump from a child focussed on childhood activities to announcing something about their "sexuality" - the parental view of a child is divorced from sexuality - they know those discussions will be had, but it's a bit of a broad side when you're not prepared. As adults we know that sexual relationships can be positive, negative, a minefield based on the millions of variables in human relationships, and the desire to protect a child from potential future pain has to be balanced with allowing them independence and making their own, hopefully well-informed choices, or following their innate orientation.
We live in a very fast paced world and have new information about all manner of things coming at us on a daily basis - keeping up is hard, but necessary