FYI - Nobody ever builds a "truly flat" roof. They are generally very "low slope" 1/8" to 1/4"-12" or so, and if installed properly, using the correct materials, they can perform very well.
In the case of the home in photo, I'll bet it is low slope, either shedding water off the back side out of view, or has interior drains that the roof is sloped to.
Many public buildings, offices, schools, court houses, libraries, large manufacturing buildings, etc. are built this way.