The Kolb's one big tube is really the only actual spar, the other tubes just form leading and trailing edges, unlike, say, a Quicksilver with its ladder frame where the leading and trailing edges are the spars.

Only the Firefly has two struts and they're not actually required structurally... Kolb only added the second strut to add drag to make the plane 103 legal on paper according to the formulas in the AC103-7 appendix.