Steve provides an argument (from Bnonn, I think) that tries to prove that those in the Roman communion actually worship Allah. The argument hinges on a statement made by the Second Vatican Council and affirmed in the Catechism of the Catholic Church.
The logic of the argument seems sound, but the conclusion is obviously absurd.
There are a few possible responses to such a reduction to absurdity. One is just to decry the conclusion is absurd and move on. This way is the way of someone who doesn't understand the significance of a reduction to absurdity. The point is not really to affirm the absurdity as truth, but to demonstrate an inconsistency. The first response is not legitimate at all.
A second way to respond to a reduction to absurdity is to challenge the reduction. Perhaps the argument doesn't really reduce to absurdity. After all, there is no need to accept an argument just because Steve Hays (or Bnonn or whoever) made it: even though he's a smart guy, we don't invest him with infallibility. This is a legitimate response, or at least it can be.
This particular case, however, may present us with a case where we should look at the third category of responses. When the conclusion is absurd, and the reasoning is sound, we need to acknowledge that there is an inconsistency or error. Why not just admit that Vatican II was fallible and wrong? That seems to be the most reasonable conclusion to draw.