Simply, anyone can drill a round hole using common shop tools. It takes relatively special tooling for a flat mortise.
The old Gibson curved truss rods had a nut and washer at each end. No peening, no locking mechanism of any kind. The rods did not spin in the neck because of the curve, so nothing other than a nut (and washer) was needed. The drilled hole is near the center of the dovetail and doesn't detract from the strength of the neck joint enough to cause failures of the tenon. When the neck joints are broken, it is nearly always the head block that splits, not the neck.
Somewhere I have a picture of a Gibson neck heel with the inset truss rod nut, but I haven't been able to find it. I can find these pictures from the process of a replica neck that I made, and they show the configuration, although not very well.
I pulled the rod out of the original neck and used it in the replica.
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