They wanted it to be rack mountable for data centers. Racks are typically 19" wide.
The case is nowhere near 19" wide. They could have made the case 19" so it actually could be rackmounted and would have had enough space to mount a (user supplied) ATX PSU inside AND put the modules in a line.
I never said they were 19". You can put something in a rack that is less than 19" with a shelf but you can't put something in a rack that is over 19" wide which the case probably would have been if they put 4 boards next to each other.
I think the thing is ~16 inches wide, leaving just not enough room for a power supply on the side (remember, there's a nasty bundle of wires, PS fan exaust, and the shelves are a hair narrower than the rack (just trust me on that). So, the PS lives on top anyhowz.
Yeah, that sucks. Looks like most power supplies are 3.39" tall so you end up over by .14 inches if the case ends up being exactly 400mm wide. The power supply could still go in front of or behind the unit but of course, then you have issues with blowing hot air into the case or getting it from the case. Hopefully gen 2 will be better thought out.