The studies cited in this article were observational, not mechanistic. What they found was that there was a correlation between being poor and having less grey

The studies cited in this article were observational, not mechanistic. What they found was that there was a correlation between being poor and having less grey matter development, not that being poor caused less grey matter development. They then speculated as to possible causes driving this correlation, but did not prove any of them in a controlled way. It would be incorrect based on the evidence they provided to say that there is definitive proof that environment is the determinant.