I am getting to the point where I no longer have a great deal of concern for what "the Law" says about anything. We've had a good teacher in disregarding inconvenient laws, and since he's broken new ground, we might as well exploit it.
When you break moral law, who cares what secular law says?
While you make a good point, I think it's pretty clear that the Senators didn't break the law the writer is citing. The terms "correspondence" and "intercourse" are used. The senators did not correspond with Iran: They sent a notice. They are not expecting or desiring a reply or to engage in any kind of negotiation. The writer is either being disengenuous or an idiot.
The law is meant to cover situations such as John Kerry negotiating with the North Vietnamese in Paris.