The abstract quite literary cautions against the way you're interpreting the data. The relevant quote:

> Notably, these gains do not uniformly translate to a rise in underlying GMA, suggesting the presence of domain-specific improvements and test characteristic changes over time. Conversely, the observed decline is primarily due to decreases in word comprehension and numerical reasoning tests, also reflecting specific abilities not attributable to changes in the latent GMA factor. Our findings further challenge the validity of claims that changes in the general factor drive the Flynn effect and its reversal. Furthermore, they caution against using these scores for longitudinal studies without accounting for changes in test characteristics.