Although Nintendo did not make shiny checks in gen 3, the only possible way one has obtained the shiny Wishmkr Jirachi is if someone (e.g. Sabresite) backs up a Gamecube sav and change the seed to one of the shiny seeds.
If you believe that externally editing a save file in order to force the game to generate a shiny is legal, good for you. I don't. These Shiny Jirachi are semi-legit at best as the shiny Jirachi was not naturally obtained. Unlike RNG abuse, they required the use of external devices. This the experiment of Sabresite's/Bond's/Kaphotic's/loadingNOW's research. I beleive he did this experiment several weeks after the post in red; I don't know his final conclusion (why didn't he edit the red post if he changed his conclusion?).
On another note, the 9 possible possible IV/PID combinations for a shiny Jirachi are known; therefore, anyone with the knowledge can create one in Pokesav and satisfy legal.exe.
This is what I gathered, so I hope it is insightful.