Nothing about this website tells me why I would want to use NixOS. It's very odd.
They're also the initial target demographics, if only because this is a limited amount of developers in what is a cross-section of two niches: Non-Android Mobile Linux and NixOS users.
But you're totally right. Once it's "ready enough" for any end-user, we'd want to make it clear "why Mobile NixOS rather than another Non-Android Mobile Linux?"
First impressions really are important.
Mobile NixOS is intended to be strictly a supserset composed on top of NixOS. So the question should be answered the same, except you add "for your mobile devices" at the end.