Well, according to this, they can't read your data.
"These folders are encrypted using only your device-specific keys and mine.
The Keybase servers do not have private keys that can read this data. Nor can they inject any public keys into this process, to trick you into encrypting for extra parties. Your and my key additions and removals are signed by us into a public merkle tree, which in turn is hashed into the Bitcoin block chain to prevent a forking attack. Here's a screenshot of my 7 device keys and 9 public identities, and how they're all related."
"These folders are encrypted using only your device-specific keys and mine.
The Keybase servers do not have private keys that can read this data. Nor can they inject any public keys into this process, to trick you into encrypting for extra parties. Your and my key additions and removals are signed by us into a public merkle tree, which in turn is hashed into the Bitcoin block chain to prevent a forking attack. Here's a screenshot of my 7 device keys and 9 public identities, and how they're all related."
https://keybase.io/docs/kbfs