Table-Driven Agent Program
You are asking two questions. The answer to your first question is very simple – the authors used a figure of speech when they wrote over $10^{250,000,000,000}$. What they meant was roughly $10^{250,000,000,000}$. In any case, the exact number isn't important – it is just supposed to be an impressively large number rather than an exact estimate. For this reason I didn't bother to check your calculation. It is possible that your calculation is right, and the authors either got it wrong or preferred writing a round number rather than a more pedantic one; but it doesn't really matter.

As to your second question, probably the authors meant what they wrote – even when the table is small, it's not clear what its contents should be! For example, consider a speech recognition program. It is possible to convert the high dimensional raw input into a low dimensional representation using a vocoder. However, it is still difficult to parse what the person is saying, for many reasons. (This is just a guess as to what they meant; you aren't supplying enough context.)

If you want to understand the authors' comments mentioned in your second question better, I suggest you keep reading the book. The authors probably offer solutions to these problems, and these solutions will highlight the problems themselves.