Why many abstract nouns get an indefinite article while it's taught that abstract nouns shouldn't get it?

I was told that abstract nouns don't get articles. (You can see here for example). But I found many abstract nouns that get indefinite articles, such as: "of a truth", "of a size", "of a lifetime", "of a certain age", "a love", "a wish", "a difference", "a power" and more. What is the explanation for that?