This map in particular was made by Gilles Robert de Vaugondy. Vaugondy and his son, Didier Robert de Vaugondy, were very respected French cartographers during the 18th century, with the son even being appointed the official geographer for King Louis XV. This is important because it means that a potential inaccuracy of the map is fairly low. However, it's also important to note that Gilles and his son were descendents of Nicolas Sanson, the famed "Father of French Cartography," and many of the maps they used were handed down to them through the family.

Now that we've established who the cartographer was, we can start to try and find the pieces we need to complete the puzzle. Right below this "déserts secs et arides," we see the region of Moldavie. At the time of the map's creation, the Principality of Moldavia was under the Phanariote rule of the Ottomans. The positions of Voivode of Moldavia and Wallachia were highly sought after due to the incredible wealth of the region and the fact that the principalities were both autonomous (they did, however, pay taxes as vassal states). At the time, the Ottoman Empire was having trouble with western diplomacy, so they relied on the Greek merchants for help. These voivodes were coveted rewards because it gave the Phanariotes a chance to trade with territories not under the thumb of Ottoman rule. I say all of this because it's important to note that this region was not uninhabited or ignored, which would further lend support to the map being accurately made. This wasn't a case of early cartography where the cartographer would guess at what was on the other side of the mountain. This was a rich and coveted area of Eastern Europe.

The region of Moldavie's (modern day Moldova) soil is approximately 75% chernozem. Chernozem is a rich, fertile soul that has a high moisture retention rate. This means that the soil itself needs less precipitation since it can, to put it plainly, get more bang for it's buck. This is ideal, because this entire region sees pretty low annual precipitation, specifically the region you've noted on the map. That area is part of the Eurasian Steppe, a sprawling ecosystem that stretches across large areas of both Europe and Asia. So, you have a vast steppe ecosystem which typically gets little rainfall, has large grassland areas void of trees, with a soil that retains moisture incredibly well. These factors can cause an area to appear much more dry and arid than it actually is.

Without extensive knowledge of the soil or average precipitation patterns of the region at that time, it's not unlikely that Vargoundy charted the area based off of a map handed down to him from his relative that had charted the area during an extended period of little-to-no precipitation that caused them to consider the area a dry and arid desert.