In The Phantom Tollbooth, the different zones of the Lands Beyond may seem really out there at first. It's true that both the Lands and their people can stretch the laws of physics, mathematics, and language in ways we may never have thought possible. Part of the Lands' magic, though, is that all of their characteristics are connected to the ordinary habits and characteristics of our own world. Traveling to the Lands Beyond is a bit like looking into a funhouse mirror, as the expressions and ideas we may rely on or take for granted are blown wide open and pushed to their limits. But that doesn't make that warped image we see in the mirror any less real.
Even though they're imaginary, the different places in the Lands Beyond are described so vividly that they seem just as real as actual landscapes.
The different facts Milo learns in the Lands Beyond aren't that different from the facts he might learn in school. It's how he learns them that make that realm so much more appealing than his own.