I doubt that the circuit was overloaded. Most 110v circuits are rated for at least 20a at 10v(2200w). Most lithium chargers output 2a (more often 1a but we'll go with 2a) at about 4.2v (8.4w). That circuit could likely have had 260 chargers connected without overloading. It just looks ugly because physically connecting many devices gets hairy looking.In regard to charging, also not overloading circuits and having multiple devices charging on piggy backed multiple extension cords.
There was a lot they did wrong with the charging setup, but I highly doubt they actually overloaded that circuit. One of those cells likely had a thermal runaway and caught the nearby flammable materials on fire. It could have happened if only one cell was on charge at the time.
They need a proper charging box, which should have some exhaust ventilation directly to the outside.
They could provide a couple nice multi-cell chargers for standard cells inside the box (like a 32 cell 18650 charger). There's still going to be some ugliness because camera and scooter manufacturers love their proprietary batteries...