Maybe they used hotmelt.
Just kidding. On gap filling, in my RC aircraft days the odd mistake may be filled with micro-balloons (little hollow plastic spheres) and CA. With multiple joints on the ribs and with most of the wing glued up it is the more practical option. I did do one guitar body with mainly CA, did the back on with Titebond. Been five years and the CA is doing fine.
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Fred
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