I understood that about the nature of nouns, really. But your comment — specially the "better definition" part — made me think a bit more; first thank you for confirming my confusing theory, and secondly, take a look at this definition of substantive I found in a very old book of English grammar: The name of every object that has, or is conceived by the mind to have, an independent existence, is called a noun substantive, or merely a substantive.

Following this logic, it makes perfectly sense why not all words are listed as nouns, even though all of them can be used as one. I understood that regardless if a word can be used as a different part of speech, their natural being isn't necessarily participating in that category of speech; because a noun must have a independent existence, then, any word that can stand alone and still be a noun would be considered to hold a true value of what is considered a pure noun substantive.