When I was a JW, 'Christendom' was basically a slur. Every other religion is false and part of Satan's world, and so on. Non-JW Christians were singled out, I think because JWs in the early part of the 20th century took a lot of abuse from them in the USA. Not unexpected, from a relatively new religious group that seemed to delight in poking every other religion in the eye.
I don't know how they would view other Christian denominations now, but they still have a mostly rigid concept of salvation. If you had a chance to hear the JW message and chose not to (or rejected it), you are doomed. I guess the new approach to judgment at Armageddon might have changed that.