A power source, like a battery (meaning multiple cells), without the means to shut down the individual cells via a fuse or switch is simply asking for trouble.
Some batteries are safer than others like the good old, heavy, lead acid batteries in our cars but these newer batteries are KNOWN to overheat and to build a battery in a mission critical application like an aircraft is beyond stupid. From laptops to electric cars and now airplanes it should be obvious to anyone who knows which direction electrons actually flow, you need protective devices at the cell level.
These are balancing chargers -- at the cell level and 4X redundant. Still, even with the redundancy I think this was more likely a runaway charging problem, although the lithium cobalt oxide chemistry is more sensitive than lithium iron phosphate would be and only slightly higher in energy density.