They certainly procreate among each other. The question is whether they procreate with the non-altered species. What they say in the article is that there was a positive correlation between isolating mating behavior and the different alleles used. In other words, they still mated, but less so. When that is combined with adaptation to different environments, mating might be less.
The existence of a gene that both creates pheromones (or influences its scent), and makes it possible for the fly to thrive in a different environment, is very interesting. In this case the single gene mutation would amplify its pressure towards speciation in the founder population that finds itself in the new environment.
I'm just wondering what this experiment did, exactly. IOW, what made this particular experiment worthy of being called "the perfect experiment".
Myself, I'm more impressed by the yeast experiment, where they took two similar species of yeast, and reversed a stretch of one species' chromosome to match the arrangement in the other, and found that they could mate & produce 30% fertile offspring (up from 1%). This showed that a single mutation can go a significant way towards speciation.