Then why didn't you find out? Start here.
http://www.nytimes.com/2012/08/24/opinion/do-american-sikhs-count.html?_r=0
you're suggesting that after I read your statement that I should go research what you might have read to form your opinion
I read the article. I don't see how it supports your belief that he is claiming that the threats against Sikhs are only coming from white supremacist groups. The article was written after a right wing white supremacist shot up a Sikh temple but he didn't say this was the only threat or even a primary threat to Sikhs. For example, he mentions in the article how they are also targeted in their own homeland in South Asia where I assume you and I can agree there is likely not many white supremacist groups?
Their distinct religious identity (uncut hair, turban, beard) has historically marked Sikhs, particularly men, as targets for discrimination, both in their homeland in South Asia and in the various communities of the Sikh diaspora.
so again, I don't understand how you formed the opinion that he is suggesting that these groups are the primary threat against Sikhs
Nothing I've read in the article you posted supports this opinion