"Property is theft" is a ridiculous slogan.
The weak point is that it doesn't distinguish between private property and personal property.
I'd define the border between private and personal property as one of "is this object or area being used in my personal life" - If yes, then it's probably personal property. If not, then we can call it private property.
The latter is what an anarchist society must abolish.
The ability to own things that other people use to do stuff is harmful.