Not saying you're necessarily wrong, but I find this hard to believe because the majority of the area was not uninhabited swamps back during the time of the Roman Empire, so why would it have become uninhabited swamps at some point between then and the 1940s? Of course terrain does change over time, but I've never heard of the Levant turning into swamps in post-Roman times.
https://www.amazon.com/Zionism-Creation-Society-Studies-Hist...
For a couple of hundred years prior there were tens of thousands of Jews in the region, including at one point 30,000 counted just in Safed by the end of the 16th century.
Also keep in mind that in 1800 populations were an order of magnitude smaller than they are now.