In the phrase you quoted, "custom" means "different from the default". It's about a choice between building customization into the browser (in the form of preferences) and relying on extensions to provide it. And it does apply here: there is no preference to honor autocomplete=off. If you want your Chrome instance to do so... find an extension.

(Though, since my quick search of the Chrome Web Store found a lot of [presumably no longer necessary] extensions to disable autocomplete=off but none to do the opposite, you might have to write it yourself. It should suffice to change "off" to an unrecognized string, as mentioned in the last comment on the issue report.)