(The chair is for concealment and steadier aim. It’s not bullet proof cover, no matter what movies show.)
An increasing number of massacres of unarmed civilians are now recorded. Most people who are able to hold a cell phone steadily enough to get a recognizable video would have no trouble holding a pistol or a carbine well enough to return fire on terrorists. And, with sound suppressors being essentially over the counter items in France, the attackers wouldn’t be able to see or hear the source all that well with the noise of their own guns masking it.