Turns out the salt flats aren’t flat.  They’re level:

The true level is a curve conforming to the surface of the earth.

-A Treatise on Practical Surveying

The Bolivia salt flats have been meticulously measured:

The DEM shows a complex surface with an elevation
6 range of only 77 cm over 50 kilometers – from a low of 3696.87 m in the broad
7 depression along the southern boundary to a high of 3697.64 m at the northeast corner.

See the article in Nature: https://www.nature.com/news/2007/071130/full/news.2007.315.html

Read the results here: Topography of the salar de Uyuni Bolivia from kinematic GPS

Here’s a lovely photograph from the salt flats in Utah:

Another photograph by David Cook: