1. Supposedly the player's character in the game was a US soldier stationed in Afghanistan before the events of the game... so I doubt 9/11 was forgotten or never happened. Besides, Russian international airports have had tight security since the '70s since they faced a greater volume of terrorist threats up until this decade. Islamist terrorism wasn't exactly invented in 2001.
2. A hundred soldiers trained for the job could probably take on five guys, body armor or no, civilian shields or no. Airport security isn't the kind of thing trusted to draftees, and the Russian army is famous for its success in hostage situations if nothing else.
Would some people die if some well-armed terrorists opened fire on a crowd without warning? Yes. But the terrorists wouldn't be able to walk through killing people unopposed, and would never leave the airport on their own feet.
2. They came out of nowhere, guns firing off in every which way, and took the enforcements by surprise. By the looks of it too, they were spread out in a wide area as well. Even worse, they were up against trained militants anyway; with superior weaponry as well. Although there IS no answer for why they were talking their sweet time; presumably, it was because they were cocky little bastards.
I didn't see any security forces present in this mission. What I saw looked like five civilians that had drawn their concealed handguns.