I think the article in the OP made one particularly interesting point - but its a shame no one seems to have picked up on it.

That being that Islamist terrorists tend to be more random, designed specifically just to cause chaos and maximum casualties - whereas far-right extremists tend to be more targeted. Though the Oklahoma bombing seems to be an exception.

Also the data is specific to the US situation, which is quite different to the situation faced in many European countries. For one thing, with few exceptions, the American Islamist threat comes from foreign imports - basically parachuted in for no other purpose than to commit terrorism. The 'native' muslim population in the US is actually very well integrated. Whereas the Islamist threat in Europe tends to be home grown - where the muslim populations there are less integrated into society.