“couldn’t put the effort in to give his spelling a quick double-check”
This writer doesn’t really know what they are talking about. Zodiac intentionally put many mispellings into his letters to throw off the authorities, perhaps to give an impression that he was an uneducated person so they would be put on the wrong track.
It’s also a common technique in cryptography to insert intentional mispellings into the plaintext, because this increases the difficulty of decrypting the code, while reducing the readability of the plaintext very little.
Not very smart to misspell the same word each time.
One of the reasons for 133+speak and lolspeak is to confound the keyword sniffers.
Exactly. And one of the details that really helped Britain’s World War Two Codebreakers was that every message ended with “Heil Hitler.”