It's not just hydroxychlroquine, the best results in the (albeit small) study were when it was paired with a Z-pack. A WSJ editorial had also talked about the same thing, but the Forbes article you posted has more details:

"The just-released French study reported that 70 percent of hydroxychloroquine-treated patients, or 14 of 20, were negative for the virus at day 6, as were all six patients who were treated with hydroxychloroquine and the antibiotic azithromycin (which Novins also received). But the study was small – 20 treated patients and 16 controls – and had other serious limitations."