iOS doesn't show messages on my phone lock screen until it's authenticated me via Face ID. Presumably on a theoretical multi-user iPad, it'd do Face ID and then show me _my_ lock screen messages and not anyone else's who happens to have an account.

Sharing a computer doesn't mean keeping personal information off of it, it just means you want a device that respects your personal information and keeps it private (non-readable by other user accounts) by default.