Apparently nobody knew the DASH8 was on the runway because it doesn't have ADSB. Hmm...
That's a mis-understanding of how ADSB is used.
There is full ADSB-out whereby an aircraft transmits its lat-long when interrogated by a SSR radar source. This is what things like FR24 rely on to track aircraft. But there is also an earlier standard (Mode S) where the aircraft replies to normal Mode A/C/S SSR interrogations, but does not respond to ADSB position requests.
Most military/government aircraft do not respond to ADSB position requests, and hence don't show up on FR24 and the like. They will show up on the airports own SSR radar screens due to the Mode S responses.
There are some 'open world' tracking sites such as ADSB Exchange that support something called MLAT. This allows non ADSB-out aircraft to be located by triangulation. However, for that to work it needs at least 3 receivers to be getting signals from the aircraft, and in that area of Japan it appears coverage is not good enough to track anything by MLAT below about 1500'. So the coast guard plane doesn't show on any publically accessible trcking sites.
Just because you and I can't 'see' it on FR24 does not mean that ATC cannot see it on their screens.
I'm well aware how ADSB works. I was making a comment on the quality of the Sky reporting that postulated the above, and I'd be very surprised if any aircraft, military, civil or other, was allowed to operate from an international commercial airport without some form of EC.