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.