Someone asked me and I'm not 100% sure of the answer.
My guess is "Export All" only exports Workflows, Commands, Finders, Filters, Functions, etc (i.e.; everything under the Designer tab) and does not include things under Execution tab such as: Credentials, Data Sources, Users, Where-as a Database Backup includes everything (and a restore replaces the complete environment with the backup).
Export All > Exports all USER CREATED CONTENT ( and not those shipped out of the box ie certified contents) under the DESIGNER Portal which is nothing but workflows and its building blocks (ie Workflow, Finders, Filters, Commands, Templates, Functions, Dictionary Entry, Custom Data Source )
Database Backup > Includes everything ie, DESIGNER, EXECUTION and WFA Configurations