Taleb isn't saying that this event can't be explained ("simple debt implosion"), or that no one predicted it. He's saying that the markets as a whole didn't take into account what ended up happening.

This crisis is built upon the fact that people thought they could predict mortgage repayment rates, and they didn't expect a major decline in housing prices. Huge parts of the world economy took these bets. The unexpected happened. Hence the black swan.

In some respect, what Taleb is saying is obvious - markets aren't predictable, and it is unwise to place bets that will kill you if your predictions are wrong. Too bad more people didn't do this.