There was very little contact on that play & it happened after the ball was in the air so there's no way it could have been defensive holding... it's either a non-call or pass interference (plus the ball was completely uncatchable).
It seemed pretty clear that the refs didn't want the game to go to overtime so the gave the Chargers a chance to win it by throwing that flag. Again, it's pre-season so it's not a huge deal, but they obviously ignored much more blatant pass interferences by the Chargers earlier in the game that had BA going crazy on the sidelines so it was complete BS to throw a flag on a phantom hold at the end of the game.
Also, you're correct that time did expire & then their QB signaled for the timeout... I don't know if their coach called the timeout on the sidelines though.