He still has a side weapon and possibly a few more tricks.
Hopefully, the cops would keep the dogs running so they don't lose the guy rather than everyone "concentrating" on the backpack.
Sac
If the Tarot shooter was of student age, the backpack theory would work well. Most AR-15 variants will break down into upper lower sections pretty quickly that would easily fit in a backpack.
The backpack would be incongruous with the guy in a deliver uniform.
My point with the graphic was to illustrate that anything is possible and everyone should keep an open mind.