Not sure why it takes so many troops. Without an exercise to go over response scenarios, the bigger disaster could be if something happened and all these people tried to respond. haha
Outside of intelligence leads, I'd say one of the attack vectors they are preparing for is a drone attack.
They are small, quiet, and remote controlled from a distance. The bad guys overseas have used them effectively against us on occasion. The trick for the attacker would be weaponizing it in a reliable way.
I'm would think some troops would be operating jammers in the typical frequency ranges used by commercially available drones. A smart EE could use alternate frequencies and maybe even frequency hopping to avoid jamming. It would be interesting to see if security also jams cell phones or maybe even turn the radios in towers off.
If an attacker could get a drone through the defenses, I wonder what type of attack they would try? One strategy would be to stay high so it is not seen or heard, but dropping a free-fall weapon from a couple hundred feet has zero chance of hitting the target. If radar is part of the security plan, the drone could be picked up. But the downside is that drone countermeasures are not super advanced. The best way may be something along the lines of birdshot from a shotgun.
Both Army and Navy have a kicka$$ radar-controlled gun system called Phalanx. It might work, but it is likely a no-go because of collateral damage from the falling rounds.
If the attack idea was to come low and fly straight at Biden, I'd guess there would be enough small caliber fire to bring it down well before it got dangerous.