Well, I always just assumed she evacuated with the rest of the civilians. Considering she, Banshee, and Rahool all survived, the damage to the Tower's Plaza probably happened later in the attack than the initial strike. When you're running through Homecoming the devastation on the outward-facing part of the Tower is pretty obvious, but the further you go in the less damage you see. There's probably a reason that the Tower didn't totally collapse after the multiple barrages it absorbed- it seems to be built into the City's wall. So, Tess probably just scattered after the initial barrage. Same with a fair amount of the civilians in the Tower. The thing about her picking up a weapon is probably just a boast.