That does not characterise a low-contrast scene. A limited range of luminance values does. So take all those colours from your triple-100-to-triple-155 sub-cube—and then add all the other RGB values that have the same luminance values as any of the points inside your sub-cube. You'll end up with many more RGB values that definitely do not fit ithe triple-200-to-triple-255 sub-cube.
You are trying to have it both ways—you keep talking about luminance when you are making an argument using RGB geometry. There is no luminance axis in this model.
What you seem to be saying is that you want to talk about colors that are not with this 100, 155 cube. I think that's what you are saying when you say "other RGB values that have the same luminance." So take [155, 155, 155] which has a L* value of 65 (in ARGB) and consider other colors which have the same luminance but are more saturated—say a LAB value of (65, 20, 0). This color is no longer in the sub cube, but will be something like [179, 143, 157]. If you include this color in the scene, your sub-cube just got a little bigger and more rectangular, but
still you can push the values up until the red channel starts to clip and you'll still have
exactly the same number of discreet values. You can't give me any set of RGB numbers that I can't add a constant to and end up with an isomorphic set so long as I don't let the channels clip.