I don't understand the problem....

With that design the TUBE IS THE "SPAR", is it not?

It's the tube that carries the load and what you are thinking of as "THE SPAR" is, in fact, just a sort of span-wise rib, right?

This is not the only design I've seen like this, I wouldn't worry about it. But if I'm full of beans and completely missing something here, I'm all ears !!