We used centimeter cubes when I was a kid. Actual plastic cubes. We could use it to add length, we could use it to add volume.

For an illustration of using them for adding lengths, a 2D certainly is fine. They know what the cubes looks like, and a 3D version would just add lines they would have to ignore. Unnecessary complexity.

As an adult who knows how basic addition and basic length measurement works, I would agree with you. The cubes are simplified to squares, that's the correct name.

But for these kids that doesn't know the difference of the terms and don't need it yet, it's another simplification to avoid unnecessary complexity.