You are correct. If a Pokémon is traded from a gen 2 game to a gen 1 game it will display that the Pokémon was met in Kanto. This is due to gens 1 and 2 not having data for what game the Pokémon originated in. Lastly, the Pokémon will still be 100% legal since this can be done without glitches or hacking on native software.
But Jirachi's backstory is tied to Tanabata, which is why it makes sense to distribute it during Tanabata.
Still, I don't disagree.
While Genesect was distributed last year, and Diancie the year before,
It'll be good to get one of them before this generation ends.
(Maybe they'll show up as Pokescraps in Japan, lol)
Because it happens every year. Some Pokemon weren't distributed since many years in Japan like Genesect or Diancie.
And Jirachi got all attention it desserves already.
Just think about it. Pokemon Games (main and spin-off), the consoles that plays those games, TCG, Ga-ole, figurines, plushies. The list on the spendings go on...
They'll learn about Pokémon, as they see me playing it.
Then it'll pique their interest, and they'll want to join me.
Then I'll have them hooked for life!
Get your kids hooked on Pokémon, and they will never have money for drugs...