They don't want to implement something that's only effective against low-level attackers because it would give their users a false sense of security and wouldn't be robust for situations involving any mid-level or advanced attacker. It would also be quickly documented so low-level attackers would be able to find out about it easily once it became common knowledge.

There's a lot of discussion about it you can search "deniability" on their GitHub issue tracker or socials.

You could allow the user to define an arbitrary number of pins with and allow the user to select which app to show for each. Then you could never know if you got everything if you want to check the phone.