As with many words, the JW's have their own, slightly different, definition for "Christendom". To a JW, Christendom is "the churches," i.e. those religions that claim to be Christian, but are not in harmony with God's will (in other words, not under the control of "God's organization"). As such, "Christendom" is seen as the dominant part of Babylon the Great, the "world empire of false religion".
In the JW interpretations of Old Testament prophecy, mostly anything or anyone who was good is seen as a prophetic type of their organization, and anything or anyone bad is typical of Christendom.
Another quirk of JW's in their view of Christendom is that they tend to view all of Christendom as being identical to the Catholic Church - ritualistic, giving church tradition equal value with scripture, heirarchial, etc. To listen to JW teachings, you would never know that there are Christian churches where the Bible is valued above all, and studied in depth. You would never know that such a thing as sola scriptura was ever thought of.
Tom
"Gentlemen, he said, I don't need your organization, I've shined your shoes, I've moved your mountains and marked your cards. But Eden is burning"
--Bob Dylan