The use of that phrase definitely brings questions. I don't think Cory is referring to America as a statistical outlier, so it must be radical Islamic terror. I am not a statistician, but my understanding is that a statistical outlier is a data point out of the expected range, which is usually discarded from statistical analysis. In that context, it might be reasonable to discard the number of fatalities in the WTC attacks on 9/11. Including the 9/11 victims does skew the curve considerably, and could cover up other emerging trends.
So, with that as a given, we might expect the terror threat from any population group to mirror their percentage of the total population size.
Pew gives the percentage of Muslims in the US as near 1% of US residents in 2015 http://www.pewresearch.org/fact-tank/2016/01/06/a-new-estimate-of-the-u-s-muslim-population/
It is harder to find hard numbers of White supremacists or White separatists. If you assume that all Evangelical Protestants belong to those groups, Pew puts them at near 25% of the population. But lots of Evangelicals are POC, so I don't think that is a great number. Also, there are Neo-Pagans to consider.
And Black separatists are not being mentioned.
It seems to me that one of the biggest problems here is that there is no easily searchable database of incidents sharing an agreed-on definition of terrorism.
New America has a page that might bear scrutiny: https://www.newamerica.org/in-depth/terrorism-in-america/what-threat-united-states-today/