That article still only uses "a powder is..." in the definition of the word sense, without the adjective, and it is a very sloppy contraction for "a kind of" or something similar. But powder is still a "material" of infinite little pieces, you need a measurement unit to count it if it stands alone as an object. Otherwise how much is one powder? (a vial of or a heap of, etc...)
It would be fine as a predicative part in a full sentence, like ..sugar is a white powder that... but it is a very different use of the indefinite article.
"A powder" in English is just used as a general thing, isn't it? One type of powder; for example, the sentence "There is a strange powder on the floor." It doesn't specify an amount of powder, eg. "100 grams", but a number of powders - in the example above, there is only one strange powder on the floor, not several different ones. As a native UK English speaker, "a powder" makes perfect grammatical sense. The way you are phrasing it makes it sound as if you think "a powder" should refer to a measurement. It doesn't; it refers to a general amount of one type of powder.