I maybe wrong, and will stand corrected if so, but he is basically just cutting to length and popping it together, and overlaying gussets and skins, and it all this goes together like a big kids meccano set, sorry for that analogy, if on the other hand there were welded joints, extreme rolling, or bending happening I'm nor sure how all this stress is introduced. As Pauline once said, Please Explain.
Where previously I mentioned ally angle being used in some home builds, vertical and diagonal bracing is pop riveted to the longeron and becomes a fairly rigid connection, in his method that mm or 2 mm as he says is a concern as movement can occur at both ends due to the brace not being connected directly to the longeron, also a smaller TEE gusset top and botton would probably be all that is required and further reduce weight and not subtract strength, but it can be said his top to bottom gusseting does add further strength but a trade off for weight, something I have noticed over the years with home owner builders is that they usually build over strength unnecessarily and maybe the case here.