You don't link without .html then you won't break anything. That's what the author is saying.
In general trying random URLs and them accidentally working and then not working, despite you weren't linked from somewhere is not something that counts as a broken link.
Say for example you added "?page=123" to a URL that had no pagination. So the normal page opens but it ignores the parameter. Then later the parameter is added, so when you add this parameter now you get a 404, because there's no such page. Was a URL "broken"? No.
I disagree - this still requires you to support both in future unless you are happy breaking old links.