It's referring to the scaling options under Configuration on the modules documentation page you've linked to.
If you don't have modules (or rather just have one, with the default auto-scaling using just app.yaml) then the Application Settings page has sliders you can use to control the instance-class, the max- and min-pending-latency and the min- and max-idle-instances. For multiple modules, you need to put this configuration in the yaml files - as you've done for your tasks module. (You can potentially have lots of modules - having sliders for each of them would make that page very confusing.)
When it says "must be changed", it doesn't mean that you must make changes, just that if you do want to make changes, you have to do it via the yaml files and not via that page.