... in 2017, but not in 2016. From your own link (and I agree the ADL is a good source):
If you count over the two year period combined it becomes:
White supremacy and Anti-Government extremism combined 32%
Black nationalism 12%
Domestic Islamic extremism 55%
Having said that if you look over a decade again your claim is true, but the worrying thing is that Islamic Extremism was virtually zero for years and now has rapidly spiked up. Whether that's a one-off or there for the long-term is a big unknown.