Looking at how the shelf has been lipped all around, I would hazard a guess that it is a
torsion box which rotates on a metal rod that has been passed through its center. I wouldn't like to guess the dimensions of the rod, although to support something as solid as a planer it would have to be rather substantial. The shelf itself would need to have a substantial filling blocks to allow the planer to be clamped securely.
I'm surprised the cabinet appears to have been made out of pine. I'd have thought something more durable such as maple would wear better because the weight on the rod will compress its wooden housing. If the rod could be embedded into some some sort of bearing housing, it would perform much better but that's getting into the realms of engineering

.
I definitely agree with you that the restraining lugs should be underneath the shelf, not above it. A better solution would be to cut a slot underneath the shelf and fit four simple turn catches which could hold the shelf steady from underneath.
The cabinet in the photo seems to be remarkably clean and I suspect it's probably being shown before its first use. I'd like to see a photo of it after its twentieth use...
Gill
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There is no opinion, however absurd, which men will not readily embrace as soon as they can be brought to the conviction that it is readily adopted.
(Schopenhauer, Die Kunst Recht zu Behalten)