Ohtani definitely knew more than he let on. The story wasn't consistent and moreover, it just didn't add up. You don't spend 24/7 with someone and not have some idea. However admitting to anything can have legal ramifications, and so he suffered a big PR hit instead.
That said, I don't have the feeling that whatever Ohtani did know, that it is without a doubt into accomplice territory. The bar is pretty high for going off to prison for many years. If his Ippei really was innocent it would have come out in court. Even for a few million payoff I don't think I'd want to rot in prison for 6 years.