Ficus produces natural latex. The entire plant or leaf isn't latex!

What is left from this process in the fake leaf is a mixture of latex sap and processed lignin, I think. It's certainly not only latex.

Right, but if you process it with baking soda it coagulates the latex into the shape of a leaf with some strengthening fiber in it, which is approximately the exact thing you'd do with molded fiber-reinforced latex