I think that a lot of the framing is being lifted from the bacterial endosymbionts of insects, which can also cause male killing. The bacteria is only transferred through the egg, so killing males doesn't harm the bacteria (and may benefit if the females have more resources due to the lack of males).

(these bacteria can also help the insect gain various other benefits)

But as a virus, this explanation doesn't make as much sense (since it should be easier to transfer in the sperm than a whole bacteria), and to my knowledge no benefits have yet been observed. But it's borrowing from the same framework, so I think that's why they called it a symbiont.

edit: Also technically speaking, symbiote refers to any long-term close interaction. Mutualistic symbiosis is the classic codependent relationship, but you also have commensal and parasitic symbiotic relationships as well.