But if players are never reported, because someone down the line makes sure their names never get out, then how is a coach to know?
It's very possible that Izzo never knew of this instance directly. If he wanted to know and told administration that he did so he could handle those situations and they chose not to tell him, then, yes, you could make an argument that Izzo couldn't do much.
But if he, along with administration/DIA established a "don't ask don't tell" culture about these types of situations where there's an understanding that administration handles these situations the way they did without Izzo ever knowing about it, then that's a big problem for him and a lot of other people.
Ultimately, given how well known Izzo is, how much power/connections he likely has at the school, and how well he knows his players (like any coach), I'd bet it's more likely the latter is true.
Note, the above doesn't regard whether what the girl is accusing the players of is true or not. Really, in this case, that's somewhat secondary. Obviously if it's true it's horrible. But the key (and just as scary) issue here is how the school handled it, and that's likely much easier to prove than whether the actual act happened or not.