It is helpful to think of the whole colony as a singular organism as opposed to individuals, because our understanding of individual starts breaking down at these levels
The high-school version of evolution, playing out on an individual level, generation by generation (one baby giraffe, with a longer neck than another, reaches higher leaves and does better) gets the idea across, but evolution is about entire species not individuals, and for the most part any single genetic variation isn't going to have much impact, unless it's fatal.
The other thing to note is that integration of "foreign" genetic material also happens inside of individuals themselves, e.g. the famous cases of gut bacteria or mitochondrial DNA. One general puts a lot of emphasis on inter-species competition, e.g. predators and preys, but there's a lot of cooperation and symbiosis happening at all levels too.
Given how unusual this seems to be, maybe it's better to ask how it happened rather than why it happened? If this was environmental pressure, then why don't we see the same thing happening elsewhere (was this really such a unique environment)? Maybe it's better regarded as an artifact of messy evolution - a quirk rather than a feature?
It'd seem surprising if this persists for long (evolutionary timescale) given that the mother "species" is expending resources to support a different "species", unless there is some kind of symbiosis here - a mutual benefit.