BTC Liquid Network is not decentralized and does not purport to be. It's a federated sidechain, that is... it's a blockchain that runs alongside the Bitcoin blockchain (using a two-way peg that lock real BTC on the Bitcoin blockchain and issues an equivalent amount of Liquid BTC on the Liquid sidechain) but blocks can only be added to the Liquid Network sidechain by some of the handpicked members of the federation.