I'm with questions above -- contact the person at Davidson listed in the letter for clarification.
BTW, my DD was just accepted to the YS program. We had the opposite situation: sent a portfolio and had to add test scores.
I don't think you need to produce an elaborate portfolio. We submitted a portfolio that was less than 10 pages. We sent a mix of things: an essay I let DD decide the topic and do the research; some drawings (a couple of early things and a couple of current things); etc.
You might want to ask your child what s/he feels is a good representation of her/his talents.
Best of luck with the application.
