It's alluded to in the Quanta Mag article, but it's worth repeating the information from the NANOgrav discovery paper[0], as stated in the paper's abstract:
However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with General Relativity.
So I'm inclined to think that all the "explanation" papers are premature. It's an interesting (low-significance) signal in the data, but more data are needed.