From the article:

At the start of the study, 20% of participants reported some level of smell or taste loss. After the third day of treatment, the proportion of participants reporting such symptoms in the ensitrelvir groups started dropping more sharply than did the proportion in the placebo group. At day seven, the percentage of participants with smell or taste loss was 39% lower in the group taking 250-milligram pills than in the placebo group. Three weeks after treatment began, all groups reported similar symptom scores. [emphasis added]

Unless I’m misreading this, they saw some notable reduction in symptoms at 1 week, but the benefit had faded away by the time they hit three weeks. This seems to imply that the drug doesn’t provide any reduction in long-term loss of smell (or at least, such a benefit was not shown in this particular trial).