That 20 µm is the size of the whole structure. Look at the details (like the placement of the nose), this precision is in the order of 10-100 nm.

When I wrote the above comment, I was ballparking it to be around 100nm, which I think is closer to µm than nm.

Looking a bit closer at it, I would say that the bands you see on the feet and body seems like the "precision" of the system. These are around 4 pixels wide. The 20µm ruler is 84 pixels wide, giving us 20µm / 84pixels ≈ 0.25µm/pixels.

So the band is 4 pixels * 0.25µm/pixels = 1µm. (Assuming my premises are right)