I think you misread wikipedia when it talks about its endosymbioses. Whole bacteria are found within an organlle (the endoplasmic reticulum) of Trichoplaxs.

That being said what you described does happen in a number of organisms (including ‘complex’ ones like nudibranchs): they steal the chloroplasts from the algae they eat in a process called kleptoplasty. Seeing as mitochondria and chloroplasts originated as bacterial endosymbionts that were then heavily integrated into their hosts, calling kleptoplasty a form of symbiosis isn’t that unusual.