How can this possibly be considered remotely lightweight, especially compared to something like Mithril? I count at least 12 repos on your github that seem to be integral components including a dedicated CLI!
One thing people really love about Ember is that the process for going from nothing to a working app is very streamlined. Typically, this is not the case with smaller component libraries. Personally, I think there's a market for opinionated tools on this side of the simplicity spectrum.
Another aspect of this is that more complex build tools can often do better analysis of your app and move more work to build time, improving the boot and/or runtime performance of your app. For me, I'll trade a longer npm install time if it leads to a better experience for users.
That said, all of the Glimmer packages are distributed as AMD, CommonJS and JavaScript modules on npm[0], with a `module` field and everything in their package.json. While it's not as turnkey as using Ember CLI, I hope people feel empowered to experiment with whatever their favorite build tools are.
When people say "lightweight" they mean small sizes over the wire. The final size of the production/deployment code. Not the development code.
In the screencast it shows that when you deploy you just get a javascript file for your components that should be super small.