A simpler way: You can avoid sampling neighbours by clamping the ray-march step so it never moves further than the current cell's boundary (plus a tiny epsilon). That way, you only cross into adjacent cells at the edge and avoid the 8 to maybe 26 extra SDF samples. (This only works if the geometry is entirely contained within each cell.)
This seems to be the article's author's own language Bauble[1], "a toy for composing signed distance functions in a high-level language (Janet), compiling them to GLSL, and rendering them via WebGL"[2].