But why quiet? Biden should be shouting this with a bullhorn so it clear to all would be troublemakers that the US has no problem going all corn-pop on them.

He just did. You think CBS News - a news-gathering division of an entertainment company and not a national intelligence agency - finds out about this so quickly unless someone in the Administration told them about it? That's a bullhorn in international diplomacy circles.

I expect the U.S. wants some plausible deniability on this, while making it crystal clear to Iran that we're behind it. That give Iran the ability not to escalate the matter. If Biden makes the announcement, Iran has to officially react - the U.S. would be officially reneging on its prior commitments. If CBS News makes the announcement, Iran can backchannel vent their anger to the U.S. and Qatar. Of course, Iran can choose to react officially and publicly anyway - but this gives them the option to engage a little more, well, diplomatically.