Zero knowledge is merely the fact that spider oak only holds encrypted backups of your files and it has no way of seeing them. LastPass tells us the same thing.
There are fair number of open source alternatives that allow you to store secrets in the cloud:
vault: https://github.com/hashicorp/vault
blackbox: https://github.com/StackExchange/blackbox
git-crypt: https://www.agwa.name/projects/git-crypt/
Pass: http://www.zx2c4.com/projects/password-store/
Transcrypt: https://github.com/elasticdog/transcrypt
Keyringer: https://keyringer.pw/
git-secret: https://github.com/sobolevn/git-secretIt's probably not worth attempting to convince certain people. Not only Encryptr, but even other ones [0].
Oh, and I'm not even remotely affiliated with SpiderOak.