For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
They have their own separate genetic code, yes, but that doesn’t make them a separate species, because they aren’t a distinct organism at all. They don’t exist in the absence of our cells.
Maybe it is semantics as you are correct but others do consider them a separate species that just happen to live in our cells. https://qbi.uq.edu.au/brain/brain-anatomy/mitochondria-what-are-they-and-why-do-we-have-them#:~:text=Two separate species became one,in every other human alive.