Jenny, as far as I can see, you are right.
I was able to get in, and when I reached the signup page, at the bottom of the form it asks you to *affirm eligibility* by checking the box stating that you are over 50 and not traditionally published. I think this must be an error. The award that requires you to be 50+ is the Karen Cushman Late Bloomer Award, not the general WIP grant.
I'm sure you'd qualify to check the Underrepresented Voices box.
There must be other people with this question. One suggestion might be to reach out to your regional group/leadership and ask if anyone else has run into this and what they did.