Hi, Lynette, it looks like she spread icing over some form of textured surface, let the icing dry, and then removed the textured surface, so you're seeing the dried underside of the icing. Our site contributor @Aproned Artist did a similar surface/texture, though on a much larger scale, in one of her recent tutorials, which you can find here: https://cookieconnection.julia...-artist-moon-landing. Aproned Artist's post should be a great help.
Aside: Have you tried reaching out to the person who made the cookies you shared in the link above? Usually the best explanation will come straight from the source, and hopefully this creator would be flattered by your inquiry and help out!