It's fun to think that someone at some point decided to just add a phallus to the drawing as an act of vandalism and that's what the giant is now best known for

It's fun to think that someone at some point decided to just add a phallus to the drawing as an act of vandalism and that's what the giant is now best known for. But note that "probably originally clothed" cites an article that merely argues the figure was originally "wearing trousers" because the phallus was not present. It doesn't elaborate beyond that. Notably "trousers" in the modern sense didn't exist at the time so the speculation about clothing seems dubious without evidence of lost outlines.