/tmp is ephemeral in the sense that it dies with the Lambda instance, but it is also persistent between multiple serial invocations landing on that instance, even if the instance is frozen and thawed between some of those invocations.

The process memory state on the instance (set up during the Init phase) is also available between invocations.

https://docs.aws.amazon.com/lambda/latest/dg/runtimes-contex...