Imagine they interview everyone who conceivably could've talked to CA during those years. And everyone says, no, we never worked with them directly. So then Zuckerberg testifies that, no, no Facebook employees ever worked with CA. But then a few weeks from now, some old emails are discovered that prove otherwise, which one of those interviewees forgot about somehow. Now Zuckerberg is on the hook for having lied to Congress! And literally no one would be satisfied that he "thought his answer was true at the time."

Lawyers see this sort of thing happen all the time. Coaching Zuckerberg not to give answers like that, would be one of the first things they'd do to prepare.