>>13026497By "studying more," you meant more years of schooling? That's not how I'd take that phrase. I'm also dubious of when the researchers start trying to massage the numbers.
https://www.sciencedirect.com/science/article/abs/pii/S0160289615000355"The interactive relationship between educational level and IQ scores is still a matter of discussion, mainly because the latter combine the influence of (1) the general factor of intelligence (g), (2) cognitive abilities, and (3) cognitive skills. As underscored by Jensen (1998), among others, it is relevant to know how much of IQ scores is due to g and how much is due to specific abilities and skills. Separating the influence of these components may shed light regarding their relationship with educational level. Here we analyzed the standardization sample of the WAIS-R for Italy (N = 1630). We considered a) measurement invariance across four educational levels, and b) IQ variability and IQ differences due to g (general intelligence), VC (verbal comprehension), PO (perceptual organization), and MFD (Memory/Freedom from Distractibility). Results showed that (a) the WAIS-R provides a valid measure of latent g, (b) individual differences in intelligence are properly measured by IQ scores at different educational levels, (c) there is an average increase equal to 1.9 IQ points in the IQ global composite score per year of education, and (d) the relevance of g decreases at increased educational levels."
So the value of g becomes less relevant at higher levels of education, which indicates they're probably not becoming more intelligent, they're just developing new skills. I'm assuming you're referring to the following study, but I can't access it to see how they attempted to rule out that it isn't just a matter of more intelligent people choosing to continue their education:
https://doi.org/10.1177/0956797618774253 
