The article starts with the statement: "For more than a thousand years, the spicy ...". I'm sure some kind of kimchi was made thousand years ago, but they definitively did not have chili back in the days. Chili must be a recent addition resulting in the kimchi we see today dating only a few hundred of years.
Gochugaru only became a common ingredient in kimchi in the 1700s (before that black pepper was used for the jang), but maybe more surprisingly napa cabbage only became common during the 1800s.
I believe the 고추가루 was derived from a Portuguese chili plant which explains the timing. Why Japan didn’t go for more spiciness is not clear. Lots of Korean recipes seem to have parallels with Szechuan cooking methods and approaches (read: lots of bold, spicy flavors) but I’m not sure if it’s merely coincidence.
No, sorry, this was told to me by a Korean colleague years ago. IIRC he said it was mostly black pepper but sometimes something closer to Szechuan pepper, which Wikipedia tells me today is probably https://en.wikipedia.org/wiki/Zanthoxylum_piperitum. His claim about napa cabbage is easy to verify (and of course daikon and other vegetables are are still very common in kimchi, so it's easy to imagine kimchi without it) but I can't any more details about the supposed black pepper version.
I read the Portuguese introduced chilli pepper in the 15th century, but Wikipedia says 16th century. There were other forms pickled vegetables like mool-kimchi but that doesn't have the spice component.
