Usually these sorts of dx are only looked at if the child is struggling in two locations (i.e. Home and preschool). For subtle differences where it is not causing difficulties in two settings, you are unlikely to get any early intervention.
These types of kids (I have one) are usually dx later - around 7/8, when their differences tend to be noticeable in comparison with their peer group.
Does this mean they may have benefited from some early intervention? Well, yes. But it's unlikely there is any on offer for kids who mask well at this point.
You are doing the right thing by being aware of it, but in a way you are hindering dx by taking protective action yourself - the fact that she may struggle more if she used her full hours at nursery would be corroboration that she is struggling in that setting iykwim.
With early-ish dx, it is a bit of a fine line between managing your child's difference and hoping they will learn coping strategies and be able to mature and deal with the usual path, or putting them in a situation they struggle with so that it is obvious they need help (and then they can get it...)
It's a fine line, and obviously the reason that these kids tend to get dx later is that it is not obvious which path they will follow - will they be able to cope if you take it slower and not need any support later? Or will they still struggle later just as they would have if you proved a need for support earlier?
It's a conundrum. Not right. But just saying how it is.
In your shoes I would continue to discuss with nursery, but I would also consider my own actions - is my own protective instinct preventing the need for support being obvious?