A deterministic wallet takes a single master seed and runs HMAC-SHA256(seed ∥ index) to sequentially produce all 256-bit private keys.
Isn't trying to find the seed of all keys basically just a little harder than breaking the key of Puzzle 256? Which means it's harder than all the puzzles combined, including the ones after 160, and then some.
saatoshi_rising once said: “By way of excuse, I was not really thinking much about the puzzle at all.”
I believe he chose a simple seed—perhaps a single word or a short phrase. And I think he was a programmer (or at least a mathematician), because he came up with the idea of generating the wallets bitwise—programmers love automation, especially when dealing with 256 wallets. Of course, it’s possible he hand-picked each puzzle; but if he did use a seed, it would be far easier to solve all puzzles with that one seed than to brute-force them.