Potential energy is the answer
You should trust the primary source more.
This is because the primary source is make its conclusion from direct observation, while the secondary source is possibly making reference to another secondary source or to another primary.
The primary source should be trusted more because it is from direct observation.
We divide the thin rectangular sheet in small parts of height b and length dr. All these sheets are parallel to b. The infinitesimal moment of inertia of one of these small parts is

where

Now we find the moment of inertia by integrating from

to

The moment of inertia is

(from (-a/2) to

(a/2))