For instance, your map devotes at least three colors to what appear to be the Slavic languages, yet only two for the incredibly diverse Niger-Congo family, even though the former is probably more tight-knit than the latter.
More than probably. The Slavic languages are a very well-established branch of the Indo-European family; in fact, most linguists consider it a branch off the larger Balto-Slavic branch. The Niger-Congo family is a primary language family with some controversy over which languages should be included and a protolanguage that predates Proto-Slavic by several thousand years.
But yeah, "Nothing ever happens in South America" applies to Africa too, so I could see the Linguistic Map Thread having one color for the whole thing.