The home page title has the word zero knowledge, I guess?
Their GitHub has slightly more info regarding this, but I agree that stuff like this should have at least a page dedicated for it that explains how they've implemented their security.
> Notesnook is a free (as in speech) & open-source note-taking app focused on user privacy & ease of use. To ensure zero knowledge principles, Notesnook encrypts everything on your device using XChaCha20-Poly1305 & Argon2.