This is a quote from your web link. This argument is totally bogus, false, and logically invalid (and the heart of his case). What Dumbski is saying is that he is so all-knowing about what is "probable" and what isn't that what he has decided cannot be probable cannot be probable. Period. What a circular argument! (second only to an Intelligent Designer being the only valid explanation for that which the ID advocate cannot understand took place in any other fashion - therefore there must be an Intelligent Designer - [as opposed to an Unintelligent Designer?]).
Notice that what he is really saying is that whatever "events" he claims are so improbable that they could not have taken place is just a dishonest way of saying they are impossible. He knows that if he said this outright he would be utterly rejected. It is either it is improbable to the point of impossibility, or it is possible.
Thus he is merely playing a word game to disguise what he is saying rather than stating what he is saying outright, thus revealing he knows what he is saying is not science. The fact that such events in question are so highly improbable they could not have taken place is refuted by the fact that they have already taken place.
This isn't evidence of Intelligent Design, it is evidence of dishonest argumentation to somehow get Creationism to be considered science under the rubric of ID.
The universal probability bound is not the invention of Dembski. In fact, he sets it much higher than most. And, it does not make X impossible, just improbable no matter what. The whole "but it's _possible_" defence is precisely what scientists are complaining to creationists about, and saying that science has error bars and all that, and then in the end to say "well, it may be more improbable than any event imaginable but you can't say it's impossible" is an abandonment of the entire establishment that was used to get away from the creationists in the first place.