Context matters. Hui likely used APL to code up the problem and APL let's one choose between 0- and 1-indexing. Reading APL papers from around the time, it look

Context matters. Hui likely used APL to code up the problem and APL let's one choose between 0- and 1-indexing. Reading APL papers from around the time, it looks common to have explicitly declared your indexing convention up front, so my suspicion is that Hui essentially was responding "I'll code up a bit of APL and get you the answer. What are your conventions?"