The researchers used a radioactive tracer to be sure that the slugs are actually producing the chlorophyll themselves, as opposed to just stealing the ready-made pigment from algae. In fact, the slugs incorporate the genetic material so well, they pass it on to further generations of slugs.
The babies of thieving slugs retain the ability to produce their own chlorophyll, though they can't carry out photosynthesis until they've eaten enough algae to steal the necessary chloroplasts, which they can't yet produce on their own.