The City represents one square mile of London's 612 square miles. The City may be one of London's two main financial hubs but it should not be confused with London as a whole.
The City of London's Planning Chief Peter Rees is talking about his area of responsiblilty, which is the square mile.
http://en.wikipedia.org/wiki/City_of_London
In terms of elsewhere in London new Skyscrapers are planned and massive redevelopments are taking place in area such as Nine Elms/Vauxhall, Southwark, Paddington Basin, Docklands, Croydon etc etc
The Shard, Beetham, Three Spires and numerous other schemes are not within the square mile which constitutes the 'City' of London
Finally it should be noted that on top of the numerous skyscrapers built in recent years within the City of London boundaries (the latest being the Heron), three further massive towers are currently under construction (pinnacle, cheese grater, walkie talkie).