On my Android I can scan the wrong finger a few times and it'll ask for my pin instead. I'm pretty sure rebooting would do the same but I'm too lazy to try that right now.
However, please make sure you try this yourself for your specific phone and Android version before relying on it.
I'm still not 100% trusting that. Any time a dev comes up with a new feature like this one, they might forget to implement a check if the game is privated (or do the check and mess up properly hiding it).