Couldnt one just use ice in a pond as a basis for a beginning perfectly flat surface? Then perhaps use clay or something similar poured onto it to create an initial flat surface and build up harder materials from there?
A good surface plate will get you 0.000001"-0.000005"
This method can take you to much more flat surfaces from first principles. Also there will be imperfections in the ice based surface itself too.
Because you could also just pour molten metal into a mold and let it cool to get an initial "flat surface" but there are limits to that.
I dont know what metal would work for that. There's still significant surface tension in an open pour. I was casting some copper a few weeks back. A ~2CM piece had about ~3mm of surface curvature to it. Plus plenty of distortion from unequal cooling where it varied thickness as well.
Differences in rate of cooling leads to uneven stresses and minute bends. Similarly while handling and working the surface plates you need to let them reach thermal equilibrium.
Large forgings which need to be machined to close tolerances are left to sit even for months in room temperature to reach thermal equilibrium.
It's usually molten tin, actually.