It's quite possible it was just recorded by a bin Laden admirer but I think it might've been intended as a morale boosting recruiting tool for new terrorists. One thing that struck me is that the tape was in Arabic. Bin Laden's guest was from Saudi Arabia and bin Laden himself lived in Saudi Arabia for years. The primary language of Saudi Arabia is Farsi, not Arabic.

Why would they be speaking Arabic? One reason might be that Arabic is the language of the Koran, kind of like Hebrew is the language of the Torah and Latin was the language used for centuries by the Catholic Church. They may've spoken Arabic because that was the language understood by everyone in the room. On the other hand, they may have wanted as many people as possible to understand what was being said.