Do you mean to say that by being "the least number not describable in under ten words" it then becomes describable in under ten words because it can be described as "the least number not describable in under ten words" which is only nine words?

Ding ding ding! That's the contradiction. And the problem is, you can't just say "Well, what's the next smallest number?" because that number now becomes the least number not describable in under ten words, which is again only nine words.

And yes, there are often ways to find a shorter way to describe a number, but by "minimal description" I mean the theoretical absolute shortest way a number could possibly be (uniquely) described.