Hawking's claim seems reasonable. It was not until the later part of the 19th century that "philosopher" meant philosopher of philosophy. Most so-called contemporary philosophers (the academics) seem to be historians of "philosophy," and they seem content to read, re-read, re-re-read Plato, Aristotle, Kant, Hegel, etc.. The irony, of course, is that no one who reads, interprets, and/or writes about, say, one of Shakespeare’s sonnets is a poet (at least not just for reading a sonnet). How does a professor of reading Kant become a philosopher? Is reading ability a qualification for becoming a philosopher? Was Kant a good reader? --It seems likely. He read Newton (that's for sure). But must we read Newton to become philosophers? Well, an introductory textbook in physics will contain more about how contemporary physicists understand the world than Newton's Principia. If a physicist were to write a book on, or perhaps an interpretation of, the Principia, wouldn't that book be classified as a “history” of physics? Yet, the philosophy section of libraries or bookstores is always filled with dissertations and/or books on (often dead) philosophers. We might wish to put the "canon" of Western thought into one section called, "Western Thought," and the commentaries on these books in a section called, "History of Western Thought." It seems unlikely that many contemporary philosophers will fit into the first section. But perhaps there is a place for the non-physicist philosophers. Perhaps contemporary philosophers are doing new, original philosophy. But isn't there this problem: haven't analysis and inquiry become scientific? That is, our world has become/ is becoming verifiably intelligible through, perhaps only through, scientific understanding. As we become more reliant upon scientific verifiability, which if you read Feyerabend isn't the same sort of verifiability that it once was ("How to be a good empiricist: a plea for tolerance in matters epistemological," from Philosophical Papers, III). It is important to note that the Feyerabends and Kuhns of the world did understand mathematical physics. They could only write what they did because they understood the mathematics. Here, it seems that philosophy provides a sort of point of view. However, it is unclear whether philosophy, in and of itself, is the same sort of endeavor as physics. That is, can philosophy stand alone? (I'm inclined to think that it cannot, and that it was not until the second half of the 19th century that a "philosopher" meant someone in the field of philosophy.)
An obvious argument against such a division would be this. “But those philosophers, who you call ‘philosophers,’ are also working off of past philosophers." Their ideas might be said to be derivative of those past philosophers. (After all, Whitehead does stipulate that all philosophy is a series of footnotes to Plato.) But such a statement would be comparable to “all mathematics is a series of footnotes to Euclid.” —That is clearly not the case; there was geometry before Euclid (as there was inquiry before Plato). The problem is that the inquiry has become more and more complex. The rising complexity is due to innovation, and philosophy has been unable to keep up. Hawking says, in A Brief History of Time that "in the nineteenth and twentieth centuries, science became too technical and mathematical for philosophers" (p.175). It might be said that the more complex mathematical physics becomes, the more question can be asked. But this complexity needs to be understood in order to ask the questions that either help illustrate a current problem or illuminate the way to solve a current problem (which doesn’t lead to knowledge, but greater complexity—more questions). (It might be that the more question we ask, the more we know--or that being able to ask meaningful questions is a sort of knowing.)
Hawking sees that philosophy has, for the most part, left behind the world--left behind that difficult language of mathematical physics--for greener pastures where the same, old, and stale grass never seems to stop growing. ("[I]n the nineteenth and twentieth centuries, science became too technical and mathematical for philosophers," from A Brief History of Time). But of course, Hawking seems to concern himself merely with the philosopher's relation to the scientific world--as though that were the world. A little later on the same page, Hawking critiques Wittgenstein for apparently claiming (I can't find where W. makes this claim) that the "sole remaining task of philosophy is the analysis of language." Hawking then adds, "What a comedown from the great tradition of philosophy from Aristotle to Kant!" Regardless of whether W. said this or not, it is clear that Hawking misunderstood what is probably meant by Wittgenstein here (or at least, the circumstances in which W. might have made such an utterance.) Of course, Wittgenstein concerns himself with philosophical problems ("philosophical problems arise when language goes on holiday," PI sec. 38). Thus, he sees mathematics as having fewer epistemological problems than ordinary language (perhaps because the boundaries are drawn with sharper borders). But, he also claims that mathematics is not ideal, it is normative (Remarks on the Foundations of Mathematics, sec. 61). Therefore, to understand mathematics is to understand a particularly useful normative language. It is likely that Hawking wants to hold mathematics, and mathematical physics, above other language; no, it is likely that Hawking wants to say something more than even that. (It is ironic, as far as I can see, that Hawking wished to criticize the direction that Wittgenstein took by essentially assuming that what Wittgenstein was aiming to criticize can be taken for granted.) But this is a somewhat different issue.