I think it is a relatively common abbreviation used in informal writing online, similar to "AFAIK" or "AFAICT" (maybe those are not so common?) but admittedly less common than "QED" or "etc." Yes, it means "as far as I understand it."
Anecdotally, I'm a native speaker, have been reading text on BBSes and then the internet for well over 30 years, and have never seen that one. AFAIK has been around for decades.
Like some other commenters, I skipped right over it rather than try to parse it.
I've been online for 20 years, and AFAIK was pretty common, although a few years ago I came across AFAICT which was pretty easy to extrapolate as ending with "can tell". From that I was able to recognize the pattern here and pretty quickly got "understand it" for UI. I think it's pretty neat that we're able to adapt language in this way, though obviously not foolproof or universal.
"AFAIUI" is nowhere even close to as common as your other examples. I have never encountered it before, and Google Trends doesn't even register it in a comparison between the others.
Trends isn't the best for this as it's search results not occurrences so really all this will show is the number of people confused by the term for weird contractions like this. "AFAIUI" could theoretically just be completely understood by the community using it.
Oh, I understood what you meant, I was just providing an additional data point for this particular conversation in response to what you were saying re: search queries vs. results.