ddd The video shows a pie that shows the ratio of the weights each vertex has, so once you know that, it's easy to imitate. In fact, what it is doing is the same as the Flour Sack Rig's belly weights.
We do not distribute the Spine project with the exact same cube, but there are a number of example projects themselves in which the 2.5D effect is used. For example, mix-and-match-pro's head is able to produce 2.5D effects with the head-control bone.
If you want to see a similar cube-shaped example, you might want to look at snowglobe-pro's present box. This is a 2.5D rig of the type that uses two bones as shown in the tip here, not the one you linked to.
The weights on the bones are almost the same as the tip you were referring to, the only difference is whether the bone weighting the vertices at the back of the box are moved by the transform constraint or not.
For a detailed explanation of the 2.5D rig when using transform constraint, just refer to the documentation of mix-and-match: Mix-and-match example #Face control
Just in case, both mix-and-match-pro and snowglobe-pro can be downloaded from the Examples section of the Welcome Screen:
