That link rather misses the problem in only considering the total population. The graphs under https://www.population-trends-asiapacific.org/data/JPN give a more nuanced view of the problem; graph 5 shows that the proportion of the population that is past child-bearing age is steadily increasing. In 1990 the 0-14 and 65+ age groups were equally large, today 65+ is three times larger. In essence the increase in longevity is somewhat masking the decline of the young.