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He tested the coding/process/whatever of the RNG himself. From what I understand, he tried to force Victini, Reshiram, Zekrom, and Kyurem into shiny encounters. For the first three, the game never started the battle because of an infinite loop caused by the game's countermeasure against their being shiny ("if shiny, skip this PID"). For Kyurem and I presume other Pokemon, it worked every time.
You know, the almost complete lack of change in shiny Reshiram and Zekrom's colors makes me think that they really did just add them in as a safety, like ??? Arceus from last gen. They're not meant to be shiny unless of course they release a specifically-shiny Wondercard of them.