Android bundles a Linux kernel, or it won't run, it doesn't expect you to supply your own. It also doesn't try to reinvent DNS or TCP, the VM bytecode format or all the other things that Android leveraged to make itself into a useful platform so quickly.

The other difference is that Android has millions of users, Urbit, not so much.

Inferno can run on bare metal, so I'd say it's different. Urbit -- I'll believe it when I see it.