I could even suggest that they poured a thin sheet of soap then cut out with a cookie cutter. But I also like the pour a very small amount into circle molds answer.
All good suggestions.
Now that you mention it they aren't all the same thickness.
But I guess that could happen with a circle mold or a thin sheet.
But, wouldn't either method result in a bent cake of soap?
When I shave off a bit of soap to straighten a loaf they curl badly.
Maybe a thick mandolin (I have one of those and will give it a try) but that doesn't solce the curling problem.
Jade - the ones on the link you gave a a bit thicker so, while not as uniform, would work really well, I think. Also that would overcome a problem with adding extra salt or something else to produce a hard bar to do a sample.
Soap makers seem to be amazingly inventive and creative!