It actually says that one policeman estimates it as less than a hundred and other criminologists put it much higher. That plus the possibility of it spreading is dangerous.
You're right that undoubtably there are Muslim gangs in the US, but by and large they are terrorizing their own neighborhoods. I've never heard of any gang paying extortion money to a Muslim gang. That said, I could see the GD's (a very powerful US gang) ending up Muslim. They aren't...at least not yet...but they are developing a culture as much as commiting crime and they are heavily influenced by prison...where Islam is thriving as well.
An interesting parallel...there is, or at least should be, a struggle between the radicals and the so-called moderate Muslims, if they really exist. Freedom House just put out a study of the literature at major US mosques. It ain't moderate at all. Scary reading.
When I lived in Brooklyn, the Arab Muslims had street gangs who were largely involved in petty crime and some drug dealing (Marijuana and some heroin as well). There is also a group of blacks know as "5 Percenters" who combine Islam with pop psychology and are involved in criminal activity. Most of our gangs in the states have no religious affiliation.