brassica is the genus, like homo. Unlike homo, brassica is not a monotypic genus. There is only one extant species in homo, that of sapiens. brassica contains many species, including several mustards, turnips, rutabagas, canola and some other things. oleracea is just one species in the brassica genus, which is all that diversity of plants previously mentioned (and we better hope that there is no pandemic that affects brassica species, as they are a huge food source for us).
In fact, we are not even a species, we are considered a subspecies, homo sapiens sapiens. As different as we all are, we are a narrow band of life, our differences being much more trivial than most species and not well served by othering.