Saving a PIN locks the app immediately. It also locks when the tab is hidden, and after the idle time you pick. Email can clear the lock if you forget the PIN.
The PIN is stored as a hash. Kissend does not keep the digits you typed. Unlock with a fingerprint or face is not part of the app today. The PIN is the unlock.
What the lock does not hide
The lock covers the open app. It does not rewrite a notification that already arrived. Quiet hours can hold those notifications until morning, and you can mute a single chat for 8 hours, a week, or until you turn mute off.
Message text in the phone's app switcher is not hidden yet. If that preview worries you, lock the phone itself as well.