For example, when someone makes a deposit (say of ERC20 USDC to earn interest, around 3.0% currently) on https://compound.finance, that data is freely available and the "receipt" becomes another token (the ERC20 USDC cToken). [1]
This token can now be used for other things, on any other protocol, without the involvement of compound itself. For example, there is a "compound" pool on https://curve.fi that allows users to deposit cTokens so that they can earn interest on their stablecoins while also providing liquidity for stablecoin swaps and earning swap fees as well on top. [2] In fact, with this pool, the user can deposit/withdrawal just pure ERC20 USDC instead and curve will deposit/withdrawal that into/from compound on behalf of the user, again, with no involvement of compound at all. (other than interacting with its "immutable" smart contract)
This deposit then gives the user back another ERC20 token "cCrv" that can then be used in other DeFi protocols without the involvement or authorization of curve.
At this point people are talking past each other because "centralization" can be used to refer to many things. The author's analogy is arguing about the direction/standardization of the technology, and the proponents of the technology are talking about the user data.