Is an 11% genetic incursion a lot for ~ 150 generations? I don't know, TFA doesn't hint, and I suspect we don't have an abundance of natural experiments to compare with.
I am not sure either. However, we do have a lot of natural experiments (and lab experiments) to explore this.
The 11% number comes from a clustering analysis. The user supplies k = number of ancestry groups, and the algorithm estimates proportion of genetics specific to each of k ancestry groups. So basically fancy k-means clustering. I don't know how robust it is and I would suspect a biased estimate.
As for over time... also complex. But I would imagine the best models would use exponetial distributions or similar as their basis (i.e. initially fast decrease in percentage followed by a slower rate of loss until equilibrium is reached).
Thats not what this data shows. What it does show is that some migrants from india joined the existing culture and left a genetic, and presumably, cultural mark. When a smaller population mixes genetically with a larger population they effectively add a small amount of novel genetic markers to the population in what we call introgression.