"Bunked off early"

Here's the BBC's diplomatic correspondent James Landale this afternoon marking a moment of history for his country:

A slice of diplomatic history at lunchtime when Boris Johnson chairs UN security council. Last UK PM to do so was John Major in 1992. That was the first time the UNSC had sat at head of state level. Also round the table were Presidents Bush, Mitterrand & Yeltsin. 

And here he is again, a little later, trying to shame his own country's PM:

So, after all that, Boris Johnson bunked off early, having chaired the council for less than an hour & listened to only 5 speakers, handing over to Dominic Raab. I wonder what the other contributors - including heads of government & John Kerry - thought about that? #diplomacy

My questions:

Is the BBC's diplomatic correspondent saying that the UK PM is lazy here? (It certainly sounds like it. And may be true. But...)

Why is he stirring the pot against his own country's PM? Why even raise such questions (off his own bat)? 

Isn't he also, indirectly perhaps, also stirring the pot against his own country's interests? #undiplomatic?

And why is he worrying about the possible offence to other world leaders - and John Kerry? And was Joe Biden there? Or didn't he bother to attend? Maybe, being a world leader, Joe's as busy as Boris, and vice versa? 

Finally: Why do BBC journalists do this?