Can't is not the same thing as won't. None of us (not even Congress) are entitled to the SEC chair's opinion on anything. The job of ruling whether or not ETH is a security is one for the judiciary, not the executive.

All of this is moot anyway. We know the SEC is bringing a case against coinbase. If that case lists ETH, then we know that the SEC thinks it's a security! Congressional testimony is a weird thing to obsess about, but each to their own.