Or you wouldn't because every kid using Google Doc would be using Smart Compose autocomplete feature and corrector making everyone's prose very homogeneous.

Yes, of course. When the red swiggle first began to manifest, I remember noticing quite a few words that I had been spelling in a specific way, often informed more or less by orthography and etymology, but still incorrectly. For those who struggled with a lot of words, their misspellings could be nearly as distinctive as their handwriting.

I think "paradice" is likely an intentional mistake on the Zodiac's part, though. His apparent level of education doesn't fit with him being unfamiliar with that word.

there's speculation that he was spelling badly on purpose to make it harder to compare to his real-life writing style

Sure. I'm a good speller but there are a few words that I learned the wrong way and had a blind spot for. I discovered I had been spelling them wrong all my life when Word kept underlining them in red.

In related news, Japanese teachers lament the fact that predictive text entry systems on their students' phones mean that they don't know how to write kanji anymore. They can recognize them, but don't have enough practice to write them consistently anymore.