To me, this reveals that Boris-whose critics said he'd be a mascot, not a mayor - is not letting the City Hall machine run on autopilot. It is a glimpse of how he gets stuck in, and doesn’t take any lip from the likes of Sir Ian. And nor should he having won one of the largest personal mandates in Europe.
CORRECTION: I rushed to praise Boris a little too quickly in my conclusion. As David Boothroyd points out below, the exchange took place long before the Glorious May triumph - so Sir Ian was chastising Boris the Candidate (a point Ch4 rather glossed over in their report). No doubt Boris is getting stuck in at City Hall - but these letters aren't evidence of it. Although, as Simon Chapman says below, they're a great example of how even then he was utterly unintimidated by the Met.