Does it mean it also runs in a browser? Why isn't there a demo?

It does look like Filament has a web target:

https://github.com/google/filament

but if they're targeting embedded systems, maybe they haven't prioritized a public web demo yet. If the bulk of the project is actually in C++, making a web demo probably involves a whole WASM side-quest. I suspect there's a different amount of friction between "I wanna open source this cool project we're doing" and "I wanna build a rendering target we won't use to make the README look better."

Someone asked in the Questions section at the end and they said No but they would be happy to discuss its possibility in a Github issue.