This is based on a common misunderstanding of Blockchain. Yes transactions are immutable, but reversing is not necessary. All fiat backed stablecoins run on decentralized infrastructure but they also operate on smart contracts that give the creators the ability to mint and burn coins at will in order to credit fiat deposits and burn tokens for fiat withdrawals. The fact that it runs on an immutable Blockchain just means transactions themselves aren't reversible but it doesn't mean new transactions can't be made to rectify a problem with loss or stolen funds.
Sure, you can make new transactions that do whatever you want, but can you force the recipient (edit: or smart contract author) to create a transaction that returns the tokens or do you have to hope they'll do it on request?
