I apologize if this is a loaded question, but... when you think about it, why exactly would we be enforcing these ISO rules?
The vast majority of the developers for the PC-FX are out of business/don't make games anymore. (Even Hudson Soft, who'd basically be the biggest one of the bunch. And Konami who owns their IPs show absolutely no interest in protecting them.)
Pretty much every exclusive release is the absolute definition of abandonware to the 10th degree.
No sales are being lost to anyone except 2nd-hand collectible resellers.
There's literally no one like Nintendo/Sony/Microsoft who'd have any interest in shutting down the site over "piracy" concerns for a failed Japan-only console that is 20 years old.
The software itself only has the ability to gain from any kind of further exposure.
And the ESA doesn't seem to care at all for niche Japanese titles.
I can possibly understand if you think PC-FX distribution could be mistaken for mainline PC-Engine ROM distribution (for which most of what I said still applies outside of some very specific software releases, but for some it's those releases that make it matter, I guess.) but I'd feel that enforcing this rule on PC-FX disk images doesn't accomplish much: it just stifles discussion without gaining anything of note in return.
Of course, it's not like there's much to gain either, since people interested in downloading the disk images probably already know where to look for anything that isn't a development-tool image. But I'd say it'd at least make for a more easy-going environment where people wouldn't have to worry about whether a subject dealing with disk images is "too taboo".