Android differentiates between user processes and root processes. I'm pretty sure iOS does as well, although maybe they've coded it as something weird.

I'm not seeing people here arguing that Linux could get by with only supporting 1 user account. I'm seeing people argue that the biggest reason they avoid running as root is just because userland applications complain about it. It's very difficult to do sandboxing if there isn't some kind of differentiation between a privileged and unprivileged process.

Regardless, Linux also doesn't really have good sandboxing by default, so I'm not completely sure what you're getting at. It's still a bad idea for people to run a Linux system as root.