The article is referring to a brand of colonialism mentioned, also next to Spencer's name, earlier on:

> The philosopher Herbert Spencer famously argued that competition among men for survival was not only inevitable but good. [...] In retrospect, it is easy to recognize his theory of nature as an unacknowledged description of nineteenth-century England... It was only natural that colonial regimes brutally extracted resources from Asians and Africans unable to resist their guns. Those who survived at the expense of others were simply proving their own fitness, and their enemies’ weakness.

Or, as I specified in my original comment, "the colonialism of his day".