"So nobody could be more surprised than I was that a Lisp dialect ..."
"So nobody could be more surprised than me was that a Lisp dialect ..."
Then it becomes clear that "I" is correct.
No, it's not.
Your sentences are correct, though, but you altered them from the original by adding a verb, thereby requiring the presence of a subject. The original sentence doesn't have a verb, therefore they need the object form.
In other words:
"You're stronger than I am"
"You're stronger than me"
This last sentence is the form used in the article.
tl;dr Being a grammar nazi is a mug's game.
Conjunctions are supposed to connect whole sentences, and while "I am" is a complete sentence, "I" is not, which casts doubt on the correctness of "than I", thereby hinting that "than" in this case is a preposition, which mush therefore be followed by an object, e.g. "me".
It's a two century old debate, but I say we settle it right now, right here, on Hacker News, once and for all.
tl;dr: "than me" and "than I am" are correct. "than I" is not.
Sandy writes better than I. (Than I write.)
I realize that grammar discussions are as boring as they come, but this is a case of someone learning English so it's important to be accurate.
It is unlikely that I'll convince you with any argument here, so I'll just recommend that you check out a reference on English usage. My favorite is Merriam-Webster's Dictionary of English Usage.
P.S. Clojure is great and I am enjoying this book so far.
Edit: Aha, I saw in a later response of yours that you're already aware that both usages are considered fine and just have a strong personal opinion. Carry on.