This just means that as far as users are concerned there is no such thing as an immutable blockchain. That's because people don't interact with chains directly, they use "Bitcoin" and "Ethereum", and those names have an inherent mutability in what they refer to.
The Ethereum developers convinced enough players of the crypto ecosystem to change what chain the name "Ethereum" refers to, not unlike a dns change (but to a large extent, done manually; there were humans in the loop to activate the hard fork). So wallets and exchanges changed the chain that the name "Ethereum" pointed to. Nodes migrated, and miners too. And it just happened that this fork was made specifically to revert a certain transaction.
Those that were left on the original chain had to pick another name for it, and build another branding. It was now called ETC instead of ETH, even though it was ETH that changed; ETC points to the old chain that used to be called ETH.
From the perspective of users, the transactions on ETH were truly reversed. Immutability is a mirage. (This is not necessarily a bad thing though.)
The Ethereum developers convinced enough players of the crypto ecosystem to change what chain the name "Ethereum" refers to, not unlike a dns change (but to a large extent, done manually; there were humans in the loop to activate the hard fork). So wallets and exchanges changed the chain that the name "Ethereum" pointed to. Nodes migrated, and miners too. And it just happened that this fork was made specifically to revert a certain transaction.
Those that were left on the original chain had to pick another name for it, and build another branding. It was now called ETC instead of ETH, even though it was ETH that changed; ETC points to the old chain that used to be called ETH.
From the perspective of users, the transactions on ETH were truly reversed. Immutability is a mirage. (This is not necessarily a bad thing though.)