The point is, that prepositioned weapons inside of a security cordon can be recovered after the assailants have passed through screening.
Pre-positioning may not have anything to do with this find. It may be normal weapons supply line for terrorists and drug cartels in the DC area. This may be a regular, ongoing activity.
Hundreds of guns have been stolen in the Atlanta area in the last 2 months, organized smash and grabs at gun shops using stolen trucks. All these guns are hot, and one would not want to be caught with a trunk full or them, so blind transfer via hidden cache would make sense.
While DC isn’t huge, it’s not tiny, either. The whole District wouldn’t be in the security cordon, just the Capitol to White House area. The Canal road area is pretty far away.