Honestly, from looking at the NTSB pictures, it looks like ALL the nuts never let go of the bolts. The picture at the top of this article even shows that it was the top-left bolt itself had fractured.
If it’s corrosion or over-torqued bolts, then I don’t think that torque checking the bolts or a visual inspection would find this kind of problem. They should at least do metallurgy on that top-left bolt for what kind of corrosion is on it before concluding that it was simply “loose”.