They should have used SOME glue, whether hot melt or CA, but chose not to use ANY glue for the box, and that is why it failed. There were two things they could have done to make the box solid, and that would be to make the plywood piece go all the way to the top and bottom of the spars and glued them in place, and there would have not been any failures. Instead the plywood comes up to the bottom of the top spar and top of the bottom spar, doesn't touch them and no glue joint to secure them. This is most likely the reason the kits were discontinued in the first place.
Here is what I find interesting, the ARF is not built the same as the kit, so who made the ARF parts? Are they based loosely on the design of the SIG kit? Does SIG make the parts?