You're right. The website right now was written for existing NixOS users.
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?"