You are right, but some people will only copy and paste this code into their own init.el -- at least I will. So I think there is nothing wrong with the word "recommended".
But I think it is necessary to choose a more reasonable key binding to add to the description.
Although I don't know which key will be more reasonable. But I don't think a key binding that is easily confused with kill-buffer is reasonable.
Given that key sequences consisting of C-c (mode-specific-map) followed by a letter are reserved for users (see (emacs) Key Bindings, I'd say there are plenty of C-c bindings which are "close" to the corresponding C-x binding. IMO there's little, if at all, we can do about this in the general case.
Besides, kill-buffer prompts the user before doing anything, so accidental mistypes have a relatively low chance of breaking much.