There's plenty of stories about professional athletes who lost millions, if not everything, because they let the wrong people manage their finances. It is not out of the realm of possibility that Ohtani wasn't at all aware of what was going on.
But...
What I find interesting is that his agent was aware of the account and asked Ippei about on multiple occasions. The agent wasn't implicated. So, that begs the question for me. What was the deal with the agent? Did he accept Ippei at face value and never mentioned it to Ohtani? That's where it gets murky for me. I would suspect the agent would've given his client a head's up...