I'm extremely skeptical of that claim. I'd buy that, on the scale of individual houses, a large battery bank could make a fire worse. Once you get to the point of whole cities burning, I just don't buy that batteries make a difference. I _might_ be willing to believe that in some narrow technical sense some homes burned hotter or faster because they had batteries. I don't buy at all that, at the big scale, number of homes burned or total damage incurred was higher.
That being said, yes, utility scale batteries do pose somewhat of a novel risk, especially as they are new and we are figuring out the engineering. A new installation in Moss Landing has burned twice in the past several months, although according to reports, the damage was entirely contained to the facility.