The matrix itself does not transform the vertices into the unit cube. That happens at the perspective division stage when the W-component is normalized.
The element -Zn*Q at (4,3) will copy and scale the Z-component of the vertex into the W-component. When the W-component is normalized, the X, Y and Z-coordinates are effectively divided by -Zn*Q*Z. That's where the perspective effect and your missing scaling comes from.