Yes, pg comes across as remarkably uninformed about philosophy. But, it is a general tendency I notice among programmers, including myself, to dismiss philosophy as vague speculation. The accusation is as old as Aristophanes' play The Clouds mocking Socrates for his questioning of the status quo. Yet you don't condemn bumbling absentminded professors to death, as Aristophanes did to Socrates during his trial. I believe the dismissal of philosophy has a deeper cause, perhaps because it poses uncomfortable questions.

Regarding PG's essay, I don't think it is fair to say metaphysics is a failed project. For instance, deeply thinking about whether things can be infinitely divisible led to the concept of the infinitesimal, which led to the discovery of calculus. If any field of math can be said to be useful, calculus surely is. None of our modern IT would exist without it, nor things like rockets, flight, material engineering, etc. Yet calculus is based on the non physical notion of infinite division that Aristotle discussed in his metaphysics.

As for Plato and math, in Meno Socrates leads a boy to understand the existence of irrational numbers, which the Pythagoreans had executed its member for discovering. So not only are ancient Greek philosophers discovering deep useful mathematical truths, they are also speaking truth to power at the same time. Imagine if math were outlawed under threat of death!