Just spotted this thread which includes plenty of comments about Japan, and "FF v not-FF", so I'll clear a few things up...
...or make everything muddier?
First of all, you can't use median incomes, via an exchange rate calculator as an
international comparison of annual income.
Buying power via a "standardised shopping basket" is a bit better. But even then, the basket is culture & location biased, so it's still just a guideline.
More concretely, I bought Canon EOS cameras - FF & APS-C - and a lot of EF and EF-S lenses from 2006~2015, which was when I had a ton of trouble with my right wrist.
Thought I'd have to give up photography....
...but micro 4/3 system literally saved my sanity regarding the creative joy I get from photography.
The point being, my annual income in those years wasn't anything remarkable, and yet here in Japan, I was easily able to buy APS-C
AND FF, and then
sell it all and move into MFT without any stress to my bank account...
...because other living costs were low compared to Europe.
Maybe not the USA? I don't know.

Maybe I'm just very good with money?
But the issue here, isn't cost.
Next point:
the article was written/edited by someone who appears to boast
(?) about having "over a decade" of experience in digital photography.
Or something like that.
No offence to her, but most of us in FoP have
decades-upon-decades of experience in digital & film photography, and know that FF is a niche market, so the headline is a tad sensationalist, albeit only about camera sensor sizes.
Additionally, as
@paddler4 so accurately points out, "for what I do" is a key phrase. FF sensors, and the bodies around them, are great "for what I do".
A Medium Format photographer will say the same.
And now that I know the pros/cons of MFT, I will also say the same.
Of course, more consumers just want something better/more flexible/more "pro looking", than a smartphone.
Oh, and
The Japan Professional Photographers' Association only has about 1,300 members nationwide, so if Canon, Sony, Nikon et al want to stay in business, "definitely good enough" is the way to go. And so-called "crop sensors" these days are, waaaaay "good enough" for >90% of photography (and videography).
So,
assuming the article is accurate, then no, it's no surprise at all that FF is not very popular here.
Cheers for now,
Simon