I'm no architect, but I think the cost savings as compared to modern techniques (but very much, if not exactly, like older techniques) is the avoidance of 1) clearing and leveling the land and 2) pouring a huge reinforced concrete slab.

Before the advent of hydraulic bulldozers it was just too cost prohibitive to clear land and pour big slabs for mass-scale housing. Instead, you would pour (or build) some small footings, and then lay some beams on the footings, possibly elevated by some short columns (or simply stones or concrete blocks). But unlike what would be common for cheap housing 50-100 years ago, building codes won't allow you to build a whole house on simple footings like that because of the potential for ground instability exacerbated by uneven weight distribution and uneven weight-bearing capacity of the soil under each footing.

But what you can do, apparently, is use "cantilevered" footings tied to a small number of deep, robust, secure pads. Those pads are what guarantee the house won't move, and all you need are some shovels and a pick-up truck, and you don't even need to level any land. You then "cantilever" some simple concrete footings off those pads using reinforced concrete beams (which I bet are probably flush with the ground and look like a gratuitous use of concrete). I'd bet money that such cantilevered footings are what are underneath many or most of those columns you see under those houses, the rest of the columns sitting directly atop the pads.