I think they're using the term Symbiotic only in sense that they're connected, meaning that if Old Gods are killed, the races afflicted with the Curse of Flesh would die.

From wikipedia;
(I know, not a great source)

"The definition of symbiosis is in flux, and the term has been applied to a wide range of biological interactions. The symbiotic relationship may be categorized as mutualistic, commensal, or parasitic in nature. Others define it more narrowly, as only those relationships from which both organisms benefit, in which case it would be synonymous with mutualism."


I kind of see it as. Azeroth, at one time, probably could have existed on its own without any problems. But along came the old gods and they "imbedded" themselves so deeply into the essence of the planet, to feed off it, that they have become a parasite that would kill the host if removed. So they can be classified as both, I guess.

Beneficial to some, while not the host?

Not really sure though, I'd chalk it up to techy speak to sound cool.

Symbiosis is essentially any process where some organism depends directly on at least one other for resources vital to its survival. For instance, the HIV virus can't exist without an animal host. If you extracted it from the body and placed it on a tabletop it would die quite quickly. A fetus also can't exist without the mother. Both the HIV virus and a fetal animal require a symbiotic relationship with their host in order to survive... however one symbiotic relationship is very different from the other.

Similar to what Transit was saying, the typical biological definition of symbiotic simply describes a relationship that affects both parties. Mutualism is a symbiotic relationship that benefits both, but it is often conflated with symbiosis itself. Parasitism is a type of symbiotic relationship that benefits one party and harms the other, so Blizzard is correctly using the terms.