So they generated private keys using a simple k = k + 1 scheme.
You don't understand how key derivation in BIP32 works and your foolish assumptions that puzzle keys have any dependency from one to another is deeply flawed.
Even though key derivation in deterministic wallets is deterministic, it doesn't mean that you can calculate a sequential key solely from knowledge of its predecessor or successor (or any other key).
In a deterministic wallet all keys appear as if they're random. Period! There is no exploitable relation between individual keys
, unless you leak a private key and the xpub, but that's another story and stupid user error. Masking them down to a certain bitlength doesn't introduce any new relation.
But you can happily continue to read tea leaves...
Post edit: you can compromise a HD wallet when you leak a private key and the xpub. This doesn't really matter here, as the xpub of this puzzle is not leaked to my knowledge. We also can't verify if saatoshi_rising said the truth or not regarding the origin of the private keys. It won't make sense to me if they lied.