I have not heard property is theft. I've heard taxation is theft, but property rights are something anarchists really admire. So I don't get where that one's coming from.
"All property is theft" is a well-known anarchist adage. I first heard of it when I was like... in my earliest introduction to anarchist ideas. I am genuinely surprised you have never heard of it.
Are you sure you don't mistake anarchists with libertarians? Anarchism is all about removing accumulation of power and giving back power to the people. And huge wealth is power in the hands of few.
That could be because it seems like the line between anarchist and libertarian is quite thin, especially at times. Pretty sure I fall into the libertarian side of that camp because I do think that there may possibly just perhaps possibly be a need for as small and impotent a government as you can possibly create. Like maybe neighborhood watch level or something like that.
You seem to think that Anarchism is just the extreme/pure form of Libertarianism, and that it's just about more/less government, which couldn't be further from the truth.
I'll acknowledge my bias upfront, I consider myself an Anarchist, so keep that in mind as you read my comment.
American Libertarianism presents itself as being about "small government," which makes it sound appealing to people who are skeptical of authority but have a very shallow understanding of politics. In truth, it's an ideology that holds individual property rights as superior to all others. To an American Libertarian, any infringement on their property rights is the most egregious possible violation of their freedom.
Anarchism means "without rulers." Anarchists oppose all forms of exploitation and oppression, which is described as any arrangement in which one person or group exerts authority over another person or group, usually by force. They favor organizing society via "voluntary free association," which is when people make decisions together on the basis of general consent. To an Anarchist, private property (as opposed to personal property, there is a distinction) is just another way for someone to exploit another, such as when a landlord collects rent.
I also hear the word volentarist for those who think all human interaction should be voluntary and not by force. I think people like Amir Taaki might be just a little bit extreme for me, but I still agree with most of what he talks about. Cody Wilson is also another person who highly interests me because of the liberator pistol. The Monero community is extremely interesting to me because of their views.