I know it is a preposterous proposition to think this thing could create a 1/1000 version of itself, hence 1/1,000,000 3d printing, but why?

A mechanism that works at scale X doesn't necessarily work at scale Y.

Different forces and material properties are all at relatively different levels.

The one I know offhand that will mess with stuff is the square-cube law. If you double the linear dimensions of something, it's twice as big, linearly, but 4 times the surface area and 8 times as heavy. The strength of anything that depends on area (like friction) won't scale at the same rate as anything that depends on mass or volume or anything.

Not to mention I'm sure this thing can only print a few different materials. Unless those are also _all_ of the materials it's made out of, it couldn't even make itself.

Calling it a RepRap, with full explanation that that means a printer that can print itself, does make it sound like it could be a goal

But even if you could get a positioning system that small, I'm pretty sure you can't scale resin printing down three orders of magnitude smaller than this printer. Maybe a tiny CNC?

Or more likely they refer to eventually printing the mems devices used in the printer

The history of precision machinery is a steady ladder of using old machines to produce new more accurate components, and then new more accurate machines.

But simply making something smaller is not synonymous with it automatically becoming more accurate or capable of achieving higher resolution. If anything, very high precision machines are typically larger in size than the less accurate ones.