He's never been that prolific. I guess he writes when he's got something to say. Some of his previous essays reflect long-held beliefs, supply of which is clearly limited.
Given the length and quality of his posts I'd consider this incredibly prolific for a single writer. Otherwise I completely agree with you.
(Some time ago I made a table with the length of the essays vs time. I never published it, but if someone is interested I can do so.)
The answer to this kind of remark is always : please do so :)
I put the CSV on Github[1], there is also one for codinghorror.com and the scripts that I used to create the tables. Some additional word count data (e.g. John D. Cook's The Endeavor) is on my website[2].
Or maybe it's exactly that: in a blogosphere pumping content every hour every day (I can't believe at one point I even followed Scoble and Winer), I might have perceived somebody posting on a monthly schedule as "not very prolific".