I haven't used Whatsapp or Signal, but you don't login every time you use the chat app, right? The phone could have just been unlocked by the owner or the PIN or pattern guessed, assuming the keys are stored on the device.
I believe Whatsapp has made a few compromises in this regard, but obviously Michael Cohen didn't bother to use disappearing messages in Signal or encrypt his Signal DB, despite how easy it is to do.
That screen was never meant to serve an encryption role and Moxie recommends using Android's full disk encryption feature to ensure data confidentiality at rest.