Even in non-communist France these horrible concrete boxes were once hailed as the future of housing. When they were first built they had all the comfort the middle class yearned for at the time. The problem is that they degraded very fast and soon became synonymous with "poor people houses".
France wasn't communist, but it was heavily influenced by communist ideas. I believe it had the second largest communist party in the West (after Italy), and Le Corbusier was highly revered in the USSR.
> The cubic shape has the best utilisation of materials per unit of usable space. Houses must be cubical.
I think parent was describing combination of box-like aesthetics with the unpleasantness that is concrete.
I think you may be mistaken - domes are perhaps a better utilisation per unit of usable space.
Also, it's definitely not the case that houses must be cubical, for any reason(s), let alone just to attempt to maximise the ratio of materials per unit of area / volume.
Here's a postcard from the 60's for instance: https://i.redd.it/3ofea4jd6k2z.jpg