The thing you're complaining about doesn't even mention the word "dialect" and it says underneath: "Some Census categories combine multiple languages or language groups". So they're probably just doing the best they can with the data that is available to them.

I think you're right that Cantonese should be (and usually is) referred to as a "language" but the categories "dialect" and "language" are not mutually exclusive. For example, Dutch is both a language (for most purposes) and a dialect of West Continental Germanic (for some linguistic purposes).