You’re right in that it’s normally only used when quoting someone, but its usage can denote more than just a leaving-out of words: in the case of that article, the word “dog” might have been substituted from the original article in place of a more offensive word, like, say… well; I won’t say, but you can probably think of some. Other things that go in square brackets than replaced words include ellipses (I’ve no idea what the plural for ellipsis is) and the word “sic”: the former represents a left-out chunk of text, while the latter indicates that any perceived error in the document that precedes it directly is NOT a fault of the author’s, but of the originator of the quote. [sic] can also be used to indicate that a perceived error is not, in actual fact, an error at all.

Hope that helped =/