Jeff, all Arkansas stone are pricy, and all the well-known Ark companies sell quality stones, but they do not have time to lap the stones dead flat. Many are sold to knife or tool sharpeners where dead flat is not critical and relatively flat is good enough.

Many people repeat that float glass is dead flat, it is not, and not needed to be to look through it, but it is easy to make flat with a bit of Silicone Carbide and a flat surface.

Some Ark companies are better than others at flattening but still, they are not “completely” flat or defect free. Maybe you got lucky and got one that is. Lapping will not only get you flat but a uniform finish, from a known grit. One that will polish a razor bevel better and produce a finer edge.

You do not know what grit your stone was finished to and if whatever finish that is good enough for you, fine. Lord knows most of the vintage hard Arks were never flattened and are burnished smooth from years of hard use, they can produce fine edges without lapping.

It is not so much that they are dead flat but that they are completely smooth, defect free. The only way to ensure that a stone is completely smooth is to make it dead flat to a uniform surface with a known grit. With an Ark, you only need to do it once.

I would have thought that an Optical flat would show any surface imperfections, but as Tom Lipton shows in the video below it will show overall flatness but not always imperfections in steel from corrosion.

If the corroded steel tool makers flat, (12:01) were a stone, and the corrosion were stone defects of the same size and depth, they would affect the finish quality of the finish they could produce on a razor edge.
What you want to know is not so much flatness, but if there are any dips or defects. But to remove dips and defect, you must flatten. Silicone Carbide is an inexpensive way to flatten, you can use diamonds, but is more expensive than Silicone Carbide.

The translator itself, in this video is interesting. I did not know that.