The absolute strongest struts would be made of solid sheets, yes, but the difference between solid and ‘skeletonized’ sheets is minimal (you compromise absolute strength but the effect on stiffness is hardly noticeable). Nearly all the stiffness of the sheet comes from the outer perimeter and as long as you can hold that in place( via the cross-bracing) it doesn’t matter that you cut it full of holes.
Concept is similar to the difference between solid square bar and hollow square tubing. All of the stiffness comes from the outer skin and as long as the material doesn’t buckle/deform any added thickness is just extra weight.