I don’t support Boeing, but I feel like this aritcle is sensationalizing a possibly mundane aspect of the manufacturing. Like none of us know the specifications for assembly. The tolorance for those specific gaps could be generous and a key card turned out to be an acceptable gauge. Automotive industry uses a variety of Go/No-Go gauges that are 3D printed which are far outside the accuracy of Feeler guages, because in some cases you dont need the precision. If the gap needs to be 2mm +/- .5mm and a key card is 2mm +/- 0.4mm or whatever then its fine.
Overall, what I am saying is that the issues with Boeing is systemic across all aspects, but one shouldn’t be so quick to jump on details that make a eye grabbing article title.