Depending on what gets counted, "public assistance" stats like this usually include medicare and social security. (Social security gets debated because of how it's paid-in, but I'm not here to argue that. The fact is, it gets included in these stats). Those two make up the dominant consumption of public assistance to the point that things we hear about in the media like food stamps are kind of a rounding error.

Medicare and social security are collected primarily by older people. Foreign born populations have much lower numbers of people in that age bracket than the general popualtion.

Combine those two facts and it's not surprising that when you look at charts of foreign born vs native born, the foreign born population uses less public assistance.

It's a statistic that is more about age distribution than anything. Like when you one of those bad maps trying to prove a point but really just showing population density of big cities.