Everyone has gaps in their knowledge which can be things that "should be obvious" to others. If someone doesn't know something, they either forgot or haven't learned it yet! I appreciated the author's honesty here.
In this case the "writing system" is the set of typos that would occur when someone with an English and Korean keyboard layout forgets to switch off English and keystrokes what they expect would be the Korean. "Midjourney" is "alemwjsl" in that typo writing system
Yeah, I think it's an uncommon word. It's not a concept that would come up for most American English speakers, unless you're in a community that uses a language with another writing system (I think I first encountered it in a synagogue with Hebrew) or you're learning such a language.
I think I've maybe occasionally seen "translit." in text used to mark that the following is transliterated, but I could see that being easily glossed over.
I don't think so, but I grew up before cell phones and AI so I had to learn how to read. I'll leave the explanation for the rest who skip over the guillemet at the beginning like I did.