Knowing something is a word and having it be a word are two different things and a representation* of a word is not the same thing as a word and a red light is no more a word than is a slap upside the head.

*But, since it can be used to represent a word it can be taken as something standing in for that word for purposes of discussion. We must, however, never forget that the representation is not the word. The No Left Turn sign I linked to above is a representation of a phrase and one step further removed from that phrase than are the letters that make up the representation at the beginning of this sentence. The hieroglyphs are just such a representation of a word when they are in ideograph mode. We may not know what word one represents but that does not deny the existence of the word.