"Often"? I've never seen it or experienced it, and I've been a liberal since the 1960s.
The closest I've come to that sort of thing was a few vague accusations that I don't like Jews because I don't think they should be held to a different standard than everyone else in the world. But that was just rhetoric from a few people who have a really hard time thinking of Palestinians as the equal of Israelis.
Is there a link somewhere to Mr. Sowell's comment in context? I'd like to see him expand on his point and perhaps cite instances of this accusation being leveled in all seriousness. Right now his statement looks like a rather silly bit of hyperbole, probably because the quote was mined.
I think he means radical in the good way. 60 years ago blacks weren't allowed to use the same drinking fountains as whites in America. Or use the same restaurants and so on. I think he also means liberal in a good way.
Thomas Sowell is not white. He is black. I think that may be relevant. He is now a conservative. But didn't use to be. He used to be very left indeed. He just thinks that the war on racism is won now. So we can stop fighting it. For example, he is against affirmative action. He argues that affirmative action at this point will only increase racism. It used to be good. Now it is bad.