It's very difficult, but it's quite common for amateur astronomers to grind mirrors to a precision of 0.1 microns or better, using century-old techniques. (Typically the mirrors are smaller, but that's a matter of the available budget for materials and man-hours, not the metrological precision.) I think it's reasonable to assert that if a company doesn't have such amateurs on staff, it will not be able to fabricate optics to such demanding precisions. There are not currently any assembly lines for such things.

The century-old techniques are a little better now that you can use a laser instead of a candle, and there are improved techniques you can use, but amateur telescope fabrication techniques are totally capable of hitting λ/8 precision.