What is missing in this instance is authority and compulsion. An official censor has authority to ban or alter certain works - ultimately they have the authority of the state and the law behind them. In the case of a publisher changing works, they do so with no such authority - there is no obligation to make the changes, it is a choice.
Now you're right that self-censorship by the gatekeepers of a medium potentially serves a purpose in restricting what is available, but I'd still argue that there are important differences. Given in this case that the estate is involved, it's hard to make the case that this is anything other than voluntary. The word 'censorship' implies coercion. But where books are in copyright, the estate's express permission is required. Where they're out of copyright, anyone is free to make whatever changes they like.