I have short hair,  3/8"on the sides and longer on top. Is that too short to get the benefits of the boar bristles? Do they need to touch the scalp to be effective, or just go through the hair itself?

The boar bristles are there to distribute natural oils and product through the hair. The nylon bristles are there for scalp stimulation and exfoliation.

You're correct; the boar bristles aren't going to distribute the oils on the sides because your hair is too short there, but it will on the top. Your hair is so short on the sides that it really doesn't need oil distribution.

But you'll get the benefits of the nylon bristles on your whole head, and the boar bristles on the top.