I looked at their test app and nothing looks like zero knowledge to me in the settings. The closest thing I see is private vault but that just sounds an extra layer of password locking (and encryption too) but not in a way that would prevent the company itself to see its contents (confirmed here). The dev in that thread failed to disabuse the user of that notion will leads me to believe the term is being knowingly misused.

Zero knowledge is supremely annoying to implement and also very risky because if your users lose access to their private encryption key that they have to write down during signup, their data cannot be retrieved and it's gone forever. That means if you specifically were using that feature, you would know it from all the nagging during signup about those risks.

And again, there's a very simple way to test this. Just try logging in from a new device. You should not be able to see any decrypted notes without either entering in that private key or having another device be online to share it. If you're thinking maybe the private vault is a secret key only you have, just see the github issue above. It's not.

Having said all that...

I'm not advocating for zero knowledge in every service. I mentioned it because the marketing bugged me and felt misleading. I honestly have no idea if their app is good or not but it does look pretty. Just make sure you trust them with what you're putting on their servers.

/edit I'm sorry I want to make sure I'm not spreading misinformation and stumbled on this thread where the author claims they cannot read any of the users' data on their servers but then everyone else in the comments is debating whether it's just end to end encryption or some other derivative marketing term. Honestly I'm just gonna say it "I don't know". If it's zero knowledge and you didn't get a special string on top of your password then that means your password is your key and password resets should be impossible or come with a side of "losing all of your notes".