Words mean things...

....but not always what you think they do.

You can't tell whether a text is "clear" or "unambiguous" by simply consulting the text — you also need to see whether there is external evidence that the meaning of some term is something other than what you assume it to "clearly" or "unambiguously" be (whether we refer to the meaning intended by the author or the meaning likely understood by most of the author's intended readers). This shouldn't be politically or ideologically controversial. It's just a description of how language and communication works.

consider this text:

1 + x = 10

This may seem "clear" or "unambiguous," until you see that people sometimes say — and quite correctly,

1 + 1 = 10

There are 10 kinds of people in the world: those who understand binary, and those who don't.

Or say you have a contract that reads,

I will turn over to you my car if you pay me twenty thousand dollars.

Usually that's pretty clear; but if it turns out the contract is made by two Canadians in Canada, contemplating a transaction in Canada, we'd realize that "dollars" means something other than what it usually means in America.Even if the contract ends up in American court (say the parties move to California and litigate the matter in California), any American court will correctly look beyond the supposedly "clear" meaning of the text to recognize that the contract meant something quite different. Even the strongest version of the parol evidence rule would not, to my knowledge, require a court to assume that the term "dollar" is "clearly" U.S. dollar, even if that's the first reaction that the judge had to the contract before he learned of its entirely Canadian context.

Again, it's not that meaning is "up for grabs" in the sense that any interpretation is always as good as any other. It's just that language can't be understood without figuring out what it means in a particular context, and recognizing that seemingly "clear" words can become less clear — or clearly something else — when one recognizes the context. Usually the context is as you expected, and the meaning is thus what it clearly seemed at first. But we can't just categorically foreclose the possibility that a word in a 200-year-old document (or even in a relatively new document) means something different than what it clearly seems to be if we look only at the text.