The principles found in The Art of War were gleaned from experience, observation, and reflection upon large amounts of witness observations, not just 'theorized'. Technically, as we understand it, it is the summation of several authors, and while it's certainly got an ancient viewpoint, and an Eastern perspective, the concepts in it are as sound now - as they were then. Look, it's not going to give you detailed instructions about specific methods or weaponry, but it's general perspective will give you a solid AND complete framework for success on the battlefield. It's not a massive tome by any stretch - try reading it before dismissing it.
As for Clausewitz, that's much more modern, and much more specifically relevant to modern warfare, with a high degree of detail on the subject of the application of artillery, movement, envelopment and positional warfare. While the specifics are certainly out of date, they are not obsolete, all that's really changed in the scaling - the applications, and the perspective are still accurate, and in wide use, even today.
Technology has certain evolved over the course of time, but the prosecution of warfare, and the basic principals of doing that successfully, have not changed. A kills B - B kills A - and doing that with the highest degree of efficiency, speed and accuracy - while minimizing ones own losses - is still the same. Sun Tzu, more than any other, still defines every aspect of that effort.
.. I didn't see that one coming and actually you have a point there though because they still read both the old testament and new testament of the Bible today