“Donald Trump walked away from the Iran nuclear deal purely to spite Barack Obama”
Really, now? The reason Trump deep-sixed the Iran nuclear deal was because it was bad for the security and safety of the US public, and a threat to peace and security in the Middle East.
Darroch was no friend of Trump or the Trump administration, and has shown himself to be a partisan, if not an ally of, the previous administration and those serving it. They have sought to undermine Trump and his policies any way possible. And they have little or no credibility.
The reason for this latest leak is the same as that for the earlier leak, and that is to poison relations between Johnson and the UK foreign policy establishment on one hand, and the Trump administration on the other. Those doing the leaking know full well that the establishment media in the UK and US will glom on to this as flies to a dunghill.