But let it be clear, an attack by proxies will be treated as an attack by Iran.

You can "let it be clear" that an attack by proxies will be treated as an attack by Iran only if that's true. And it's not.

It's a bluff. Iran knows we're not going to start a direct war with them over Houthi attacks on commercial shipping. Because they know that, they're not going to stop supporting the Houthis - whether we "let it be clear" or not. Because they know it's a bluff. We're not going to war with Iran unless they directly attack us - which is why they don't directly attack us.

Legally, politically, and diplomatically it is different when a nation's proxy attacks another country than if they attack directly. Which is why Iran can do this. They know - correctly - that no other nation on earth will treat the Houthi attacks on international shipping the same way as if these were Iranian attacks on international shipping. Because they're not the same. Supporting a belligerent is not identical to being the belligerent.

So Iran knows full well that an attack by proxies will not be treated as an attack by Iran, and they're not going to believe us if we say it will be. And since we're not going to go to war with Iran over this, making threats to that effect is not going to actually stop them.