[tachim]

That first study does not support your claim. This is the main chart: https://i.imgur.com/pTFJkaJ.png. Note how it shows a significant response in Doublecortin+ cells only in the HRT group and not for the LRTs. These acronyms stand "for low (LRT) and high (HRT) response to aerobic exercise training". In particular, quoting from the article:

Starting with a founder population of genetically heterogeneous rats (N/NIH stock), we applied two-way artificial selection based on the magnitude of change in running capacity after completing 8 weeks of standardized aerobic treadmill training. After 15 generations of selection, rats bred as HRT increased maximal treadmill running distance from 646 to 869 m (change, 223 ± 20 m), whereas rats bred as LRT decreased from 620 to 555 m (change, −65 ± 15 m) after completing the same absolute amount of training (Koch et al. 2013).

This is a huge confounding variable. There is a very good chance these cells have an impact on these rats' ability to run long distances after training, so it's completely reasonable to expect that the cell growth would respond well to the very exercise the rats were selected for responding well to! It does not show that the same effect would happen in a normal rat (the LRT rats did not see a difference in this particular cell growth, but they were also nonrandom).

[nopinsight]

Your point regarding the first study I cited seems to be a good one. There are a number of other studies with similar effects, however; some of which can be found in the review article.