AFAIU, this default value for the threshold means that for lines to be detected, they have to be at least as long as the smallest image dimension (i.e. lines have to cross the entire image, and mere segments are not detected). When stepsize is 1, the value of the threshold should roughly indicate the minimal length (in pixels) that a straight segment should have for it to be detected. More precisely, it should correspond to the minimal number of pixels that have to be aligned for a line to be detected (so that in a noisy image, or if line segments are “wavy” and only a fraction of the pixels are aligned, the line is still detected).
These interpretations come from vague recollections I have from of a previous life, when I implemented a Hough transform in OCaml for a CS class… So please take it with a grain of salt!