Author here – can't say I agree. If this is FUD, I would assume there is an app you're familiar with which you can say with actual 100% certainty is end-to-end encrypted in a leakproof way?
This is why I like what the folks over at Notesnook are doing with Vericrypt [0], which essentially allows users to verify the encryption themselves in a very easy to use way.
It is a very common pattern that an app which claims to be secure turns out not to be. In fact I would say that claim to be secure and actually live up to that are probably the exception rather than the rule.
This is why we self-host 99% of the content related to managing Supernotes (as a business) on Supernotes itself, but (a small minority of) sensitive content we keep in 1Password, which is one of the few apps I trust to have done E2EE well enough for me to feel that data is secure.