Its only custodial for the lessee, for the lessor (stake holder) they hold the keys at all times so the stake is always safe. The stake is the largest amount, the lease payment escrow is small in comparison. Example circa 3 mil in SP active leases exist atm but the escrow held between the old minnowbooster and dlease is less than 50k steem, that’s about 1.6% of the total value that isn’t 100% decentralised due to escrow.