It's also possible that, so as not to compromise the structure of the (already thin) metal by drilling holes in the risers, the stair treads themselves are attached to a carrier of some sort that's then fastened around the metal structure we can see.

I can't really see this staying intact very long as is. The metal uprights are already deforming slightly while just freestanding, so my suspicion is that there's an additional structure that spreads the load across the whole assembly and which has yet to be installed. Seriously, those lower steps would just sproing and dump you on the floor scooby-doo-trapped-staircase style the first time you tried to go up them.