I think you’re probably right. Then again, it all depends upon the official definition of a Solid.
So far, I cannot find a detailed definition of a Solid in the User’s Guide or the Knowledge Center.
All I find is this: http://help.sketchup.com/en/article/1011019
Is there a better description of a Solid somewhere in the documentation that I’m missing?
Then there’s the Solid Tools video tutorial.
It begins with a demonstration of what is and is not considered a Solid.
At 1:12 the video presents an example of what is not a Solid that is analogous to your cube example.
Unfortunately the Entity Info box isn’t visible in the video.
It would be interesting to know what it had to say at that moment.
-Geo