More like...
Ok. You know what grey goo is right? It's a self-assembling swarm of nanomachines that alter their environment, replicate more nanomachines from raw materials, and construct artificial tructures out of themselves, based on a core program.
The grey goo catastrophe is when such replicators interact with an organic biosphere, and they wipe it clean, breaking it down for raw materials.
wikipedia on grey gooAccording to the MTG wiki, the phrexians have a convoluted artificial structure as their home. On the outer layer of this structure, synthetic evolution takes place, with synthetically derived lifeforms. The human-like protoforms of the phyrexian populace are often "abandoned" on this surface for testing, before "completion". These protoforms are called newts, and are grown in tanks. They have no gender, nor body hair. Other layers of their artificial habitat artifact-world serve various industrial and social functions, including a prison...if you can call it that. Its more like a hell reserved for torturing organic designers of machines, when encountered. (Artificiers).
The "oil" that permeates their world instead of oceans has mutagenic properties, and is directly implicated in the conversion of newts into phyrexians proper. It has all the halmarks of programmable grey goo, issued build order construction commands to "restructure" the newt. It also appears to maintain they cyber systems, and the integrity of their realm, as best it can. The one pulling all the strings is essentially a transhumanized intelligence, who's whims are given axiomatic truth status. It is this aspect that causes the potential exploitation I mention.
The phyrexians believe that this intellect is fundementally incapable of error. This is why only machines engineered and diected by this intellect are sacred, and why all other machines and artificiers are abominations that must be destroyed.
This intellect was halted, and destroyed.
When this happened, the phyrexian forces were fundementally demoralized.
However, the grey goo itself was not halted. One of the combattants in the conflict to destroy the tansuman intellect responsible for the horrors of phyrexia happened to have a cybernetic heart of phyrexian origin, and as such, contained grey goo. Unwittingly, when he created his own world of argentum, (amusingly, another mechanical type artificial realm) he released some of it, and it contaminated the service golem he left in charge when he left to explore the multiverse. This contamination has culminated in the independent and directionless re-evolution of phyrexia on that realm, later called mirrodin.
I presume that ZTG is referring to this new re-evolution of the cyber plague, and not the original incarnation.
In the scenario I outlined, you collect a sample of raw grey goo, contained inside a sentient artifact from both worlds. You then use them to cross contaminate the afflicted planes.
The original programming in the original phyrexians would identify the new, independent evolution as being "profane", since it came into being without the direction of the creator. It would seek systemic destruction.
The new evolution synths would consider the old synths to be obsolete, inferior, and seek to recycle them for raw material.
By cross contaminating both worlds, you afflict them with an autoimmune disease at the fundamental level. They are fundementally the same thing, but following nearly identical but different axioms. It would be like transfusing blood from an incompatible donor, and causing graft vs host disease.
The resulting conflict would demolish all higher order constructs that rely on the grey goo, because the conflict would ultimately be waged on this fundamental, pre-sentient level.
Both worlds would become reduced to a roiling sea of nanoreplicators making copies of themselves, and deconstructing the rival strain. Both would evolve in equilibrium, and prevent the emergence of higher order structures.
Without larger data storage devices and converted organic hosts, the phyrexian "race" would go extinct.