It is definitely more secure than a single node and a backup - there are many full nodes. I'm not sure what you mean by having the blockchain disconnected, but the key thing here is that no single player can change what is recorded on the blockchain ledger.
As a result, no single entity can just seize all the assets either.
Thats nice in Theory, but I think I read in Steemland somewhere that one of those decentralized wallets was hacked (parity I think it was), sooooo, who cares about decentralization when it could be hacked?
But still you say is more secure than a full node ? I personally don't know .. however it might be better than an exchange wallet, and also better than an online wallet, that's for sure ..