One key point to note is that the BackJoint and Spine bodyparts are not physically connected - it is just the maths that keeps them together. In my early attempts it kept breaking apart because I'd made mistakes in the maths !

If you look at the Hierarchy Editor you'll see that the Book bodypart (which has no physical mesh attached to it - it's just a dummy) has three direct children: PageBlock4; BackCover; and BackJoint. The rest of the PageBlocks, the FrontCover, FrontJoint, and Spine are in a chain under PageBlock4

(I'm not sure why the FrontCover bone doesn't show up in the image above - I think it's similar to the BackCover bone)

This shows in principle how it's possible, using just valueOps, to create a skeleton where the last bone joins back to the first, i.e. a loop. Of course this example only has one degree of freedom, zRotation, and 2 or 3 degrees of freedom would make it far more complex. But in principle it's possible.

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.