You are indeed correct to point out that the article is about two eukaryotes which are vastly different organisms, separated evolutionarily from bacteria by hun

You are indeed correct to point out that the article is about two eukaryotes which are vastly different organisms, separated evolutionarily from bacteria by hundreds of millions of years, or billions of years, depending on how you measure it. Similarly, humans are quite different from whiteflies, although the difference is put into perspective when you consider that we have something like 40% commonality of genes, depending on how you map homologues. How the plant gene got into the whitefly is a deep mystery that happened long ago. If we go back even further in evolutionary history, insects, humans and other eukaryotes are descended from prokaryotes. The organelles of eukaryotic cells such as the mitochondria of animal cells and the chloroplasts of plant cells were once individual prokaryotes which merged into colonies and formed eukaryotic cells through endosymbiosis. Even today, eukaryotes live symbiotically with prokaryotes in a way that is impossible to disaggregate. You have as many prokaryotic cells living inside of your shirt and pants as you do eukaryotic cells, with trillions of gut bacteria responsible for keeping you alive by pre-processing the food you buy from the supermarket and restaurants into nutrients and amino acids that your actual body can utilise. Whiteflies also have parasitic and symbiotic commensal bacteria that are integral parts of their organism. Perhaps one of those gut bacteria, or a respiratory parasite transferred that plant gene millions of years ago in a freak occurrence. Or maybe it happens regularly and systemically. With gene sequencing only becoming widely available in the last decade, scientists are just beginning to isolate and identify examples of gene transfer like the one in this article.