It’s worth bearing in mind that humans actually do have incredibly good memories, when we try to use them.
You young whipper-snappers won’t remember this, but back in the day bards used to be able to recite thousands of lines of epic poetry purely from memory, and more recently illiterate peasants could learn and recite huge chunks of dialogue in mystery plays and the like. Reading and taking notes is a good way to bring material together, but someone like Hitchens was also constantly writing, debating, preparing and rehearsing his arguments, so it’s not surprising that he honed his ability to link his general ideas to specific quotes and references. Even then, the OP’s example is from a book, which was doubtless the end-product of an extensive researching, drafting, re-drafting, editing and fact-checking process.
To put it anohter way, if any one of us were asked, right now, no prep time to give a 10 minute speech on, say ethical foreign policy, it would be patchy, error-ridden, poorly evidenced and all in all woeful.
If we were given a day to prepare, it would be better but we’d be reading from extensive notes. If we were given a week, it would be still better and we’d have a chance to memorise it.
If we then gave that speech 20 times in 6 months, with the chance to review, edit, punch-up, adorn and trim where necessary, not only would it be a hell of a speech but we could then give it apparently impromptu or - which looks better - use discreet chunks of it in conversation, drop in some quotes in other related topics and generally appear attractively erudite (which we would be, because we’d done the work).
And if we did that with half a dozen other topics, then we’d dazzle all and sundry - but we’d have to quit the day jobs, because that’s a lot of work.