The irony is that virtually anywhere in the world "locally sourced materials" now includes a staggering amount of plastic.
Considering the ocean is on the verge of containing more plastic than fish, it'd be refreshing if people started harvesting the plastic as a resource.
I have built 3 houses. Walls are among the cheapest components of a house. Stackable modules (of which there are many kinds) like these do eliminate some of the need for skilled labor and offer some advantages. My personal favorite is site-made concrete blocks. In Mexico I have seen blockerias consisting of a four block mold, a pile of sand, sacks of cement, and a hose. They are not ideologically glamorous, but they are very practical and cheap.
Seems great, though I would say the lack of insulation means they will be most useful in tropical climates. But I don't see why they couldn't make doubled-up horizontal beams that could hold two courses of 'lego' bricks with space in between (which could be stuffed with plastic bags, at that).