When speaking of something included in text, what rules make it acceptable to say "it says", rather than "it said" regardless of conditions such as age of text, or the existence of subsequent revisions of the text?
This use varies in different contexts.
In ordinary discourse we have traditionally written of past works as if they were 'speaking' to us immediately, in the present; and I think this accurately expresses how most people read novels and plays and poetry. The same use is generally followed in academic studies of literature and history when citing past critical works, which are felt to still have something to 'say' to us without regard to subsequent discoveries.
In other fields, however, past works are fixed firmly in the past: the APA style guide, for instance, calls for prior works to be referred to in the past tense, since even yesterday's study may already have been superseded.
Here's an observation. In my guidebook for Barcelona, it says that the Il Papiro restaurant opens at 11 am. If I close my guide book and put it back on the shelf and then I pick it up again next year, I am pretty confident that it will still say that the Il Papiro restaurant opens at 11 am.
In actual fact, my guidebook for Barcelona is thirty-odd years old. Twenty years ago in 1994, it still said that the Il Papiro restaurant opened at 11 am. The reason that we use the present simple for what is said in written texts is that it always says the same thing, it doesn't change. It's kind of frozen in time. We would tend to use the past tense, if this kind of text had been updated. So for example, in a new edition of a book, they are likely to say, in the last edition we said that .... But for things that are always the same we tend to use the present simple.
Notice as well that this usage of the word say is not really an action verb. It has a stative flavour. When we say that a book brother text says something, we don't mean that it is doing some action! We are describing a state or situation. If we say that a text said something, there is no particular point that we perceive the information as being said. What we are describing is that state or situation in the past.
Hope this helps!