CoinText is not that trusted, as it sounds. They do not have your key. They CoinText system has only "half they key" and needs to combine it with a hash of your number in order to get the private key on the go. CoinText explicitly does not store any private keys from anyone.

That being said, you trust them, that they "do not save your keys", because they surely could. But the idea is, that they are no trusted escrow service and also legally not seen as such, because they do not directly hold the keys to your accounts

It's explained in the Help page