He is right, although to say the only reason the empire fell was because it “rotted from within” is a vast oversimplification. There were many reasons for the decline of the British Empire - rising nationalism in the colonies and increased foreign competition being major factors.
The main reason was we were bankrupt after fighting two world wars and could no longer afford to hold on to them. If the colonies had generated wealth, you would have thought we would have hung on to them for grim death....