It’s an optional feature that’s rarely used.
There is already a default map so there is nothing extra you need to do.
The default mapping is
this.sys.add → this.add
this.sys.cameras → this.cameras
this.sys.load → this.load
etc. In that example, a custom mapping is given:
this.sys.add → this.makeStuff
this.sys.load → this.loader
Because cameras
is absent, there is no this.cameras
property, just as the docs say.