Ants that seem to defy biology – They lay eggs that hatch into another species

This is wild. But eusocial insect have a lot of bizarre eccentricities in sex determinism. less than 1% of the colony can actually reproduce, every other being is there for the betterment of the 1%. The workers will mutilate, sacrifice and kill themselves just for the queen to have 0.1% better survivability.

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

Evolution in general works at the level of sub-populations rather than individuals. Genetic variation builds up over time in entire intra-breeding sub-populations of a species (mostly isolated for whatever reason - e.g. forest elephants vs plains elephants), then once in a while there will be a big environmental change (famine, disease, new competition, etc) that may suddenly make these accumulated changes in one sub-population (different from the accumulated changes in the other sub-population) critical to survival rather than benign.

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 question is, which kind of environment pressure and context lead this ants to this adaptation. Perhaps a high mortality rate in their own species happened at some point? Maybe something like an STD?

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.