That looks to be a pretty old resource. Based on this page, it looks like it hasn't been updated in quite a few years (Jul 25, 2017). The information is probably still good, but I'd take it with a grain of salt.
I'm guessing the developer key they're referring to is what Canvas calls an inherited developer key. They're pretty handy, but each school you want to use the tool will still need to manually enable your developer key. Again, this happens after whatever review process the school has in place. The difference is that it's just a toggle switch in their admin area rather than needing to manually provision the key for you. You also get the (possible) benefit of maintaining a single client id for your app rather than one per school.
For what it's worth, the vast majority of tools I've installed in my school's Canvas instance don't use an inherited developer key and just have us do the manual steps. A rare few have us do both for some reason.