I think we assumed that GitHub users would know that gists are not private. We added a clarification to the onboard slideshow. Support for public and private repos is coming, via GitHub Integrations.
I quote from your website: "Don't worry if you don't know what GitHub is, you only need to create an account." You don't appear to make any assumptions that your users understand that their notes will be published and readable by anyone on the internet.
Furthermore, your clarification: "Currently, Kobble stores all user data in secret GitHub Gists, under your account. Secret gists are not private." does not help because it does not explicitly state the fact that notes will be readable by anyone on the internet.
Having read through the information on your site now it's been updated, it's much clearer than it was before. I feel like it should have been closer to this before submitting to HN, but the changes ("Gists are not private, and are suitable for content that you want to share", "control" rather than "own") are definitely a step in the right direction.
We are trying to clarify the multiple ways Kobble can be used. Gists are for public data that you want to share. Private repos (coming soon) are for private data. I think the intros on the web make this clearer now.
Furthermore, your clarification: "Currently, Kobble stores all user data in secret GitHub Gists, under your account. Secret gists are not private." does not help because it does not explicitly state the fact that notes will be readable by anyone on the internet.