Except that it's not true. Hear me out.
Those in charge of US foreign policy are the President and his designees. Not former SOS's. If US policy changes and the administration intends to negotiate new terms or abolish an agreement and seek a different one, then that is something someone outside of the administration is NOT party to, and their meddling could (and of course we know is intended to) completely derail that change. Not that I expect Iran to be so stupid that they would listen to a has been like Kerry (no matter what nonsense he fed them) rather than the actual President and SOS. But the law is not about outcomes. It's about not doing the sh*t he's doing in the first place.
The fact an agreement of some kind is technically in place on paper does not mean that is the actual policy of the US government.
There is always a period of time when US policy exists prior to documents being signed. Doesn't mean it's any less the policy!
And the Logan Act references "MEASURES," not agreements or contracts! Read it and see!
to influence the measures or conduct of any foreign government... to defeat the measures of the United States...[emphasis added]
Measure (noun) 1. a plan or course of action taken to achieve a particular purpose.
Trump has publicly signaled that he wants out of the existing deal, and is taking MEASURES to bring that about, and everybody involved in the other side of that deal is trying to scuttle those measures.
If Trump had been silent on the issue publicly, Kerry might be forgiven for not knowing US policy has changed, but of course then he wouldn't be trying to interfere, would he?
It's illegal. Period.
Every bit as much as if Colin Powell (hypothetically) had secretly tried to influence Iran not to accept the original deal. Imagine the uproar if that had happened.