Preschool is not the beat all end all for how she will do in Kindergarten. Plenty of kids never go to preschool at all. Unless you are noticing things at home about her behavior and you have noticed that daydreaming or districtions at home interfere with her life, you should probably wait and see what happens in Kindergarten.
I would not hold her back based on an failure to make progress in an optional program. Kindergarten teachers expect to find some children who have not learned anything formally yet at all, so don't sweat this just yet. Holding kids back is almost always harmful, even if it feels better at the time, the data shows a much different picture for them a few years down the road.
Keep a close eye out for these behaviros at home: impusivity (beyond a typical 5 year old) seeming to leave a trail of her belongings everywhere she goes. Forgetting what she is doing or saying mid action or thought. Handwriting and fine motor issues-buttons, zippers. Difficulty with mutli step directions. Easily overwhelmed by frustration.
If the issue is real, it should be addresses in the first parent teacher conference you have in Kindergarten. Ask the teacher about her atteintion span if they don't tell you anything, and keep a close eye on her milestones and academic progress. If you see something, request school evaluation, but make an appointment with either a developmental pediatrician (not your regular pediatrician) or a board certified child psychiatrist for a diagnosis.
I am very prone to suggest evaluation when ever there is a concern, because catching these things early is always best, but this just does not sound like a huge issue, because something as global as ADHD is not generally a surprise to parents when a teacher suggests it.
It is possible that she has another issue that is causing her to be inattentive. Have her eyes checked, and I would find a developmental optomitrist who can check her visual processing too (the occupational therapy unit at your nearest childrens hospital can give you a name) They will give you a regular eye exam and do some checking on her processing.
But send her to kindergarten on time, and have a great summer before she goes!
M.