“portable chargers” This item is a non battery device. An unused charger will never start a fire. A charger that uses a battery is called a battery bank. They are usually one cell. Fires can only occur during charging or discharging and in rare cases when damaged. If the luggage was ever damaged and damaged a battery, I doubt if any record was kept!
Batteries use solid oxygen as an oxidizer which is safe unless the cell over heats. This would cause the solid O2 to vaporize and could ignite. The danger will pass when the O2 is used up. A good safety method would be a fireproof bag for battery devices. The airlines could rent them if they wanted to quit crying about batteries!