I wonder if that was a mistype. It isn't unusual to find mitochondria in the cells, since all eukaryotic cells have them. OTOH, finding chloroplasts in the salamander cells would be very surprising.
Also, the article overstates the significance of foreign biological material living symbiotically within the body. Many species of bacteria live on and within the body. They protect us from infection. E. coli (and maybe other species) makes vitamin K that we need to survive. What *is* significant is that the symbiotic organism is photosynthetic.
Both mitochondria and chloroplasts are thought to be ancient bacteria that took up residence inside eukaryotes. Although incapable of independent life, they still function like bacteria.
I don't think in either of those cases that the "foreign" party lives "within" cells, but external to them.
You only read the excerpt. The full sentence is:
“Transmission electron microscopy (TEM) images showed mitochondria in the salamander cells clustering close to the algae. “