Why the collision with academic cryptography doesn't matter: Anyone who had even a basic understanding of their product + some academic crypto background would get what they were going for: they have no knowledge of whats going on. Also, strictly speaking,the academic term is zero-knowledge proof or zero-knowledge proof of knowledge. Ie, zero-knowledge is an adjective used to describe a proof (and indeed, if you look at the history of how these evolved, that is exactly what happened). You could reasonably use zero-knowledge as a modifier for something else and it could be acceptable. Indeed, it's a fairly good shorthand for a particular class of definitions of privacy/confidentiality that require any transcript of the protocol can be produced by a simulator who has no knowledge of what transpired.
The problem is Spider Oak's cloud backup cannot be zero-knowledge or no knowledge. It almost certainly leaks when you update files and when you delete them. Perhaps they don't log this or delete the logs, but they could. And this meta data could matter to businesses.