I’m not sure you understand flashlight loads vary over time. A flashlight is never going to be able to maintain a 20A load from 4.2v to 2.8v. Even if it starts at 20A, the voltage will drop and the amperage slowly goes down with it. As the amperage drops, the heat build up will slow down.

Plus, any flashlight pulling 20A from a battery is going to get really hot from the led and you will either adjust the brightness down or the driver will automatically adjust the brightness down.

The testing rigs used by battery testers constantly adjust the resistance to maintain a 20A load. This is very different from a flashlight. You have zero to worry about using that cell in a flashlight.