Okay so I was overly general with my first statement.

No one is saying passive radar systems don't exist. We've been using them for decades and there have been massive improvements. But the actual radar system we're discussing here isn't. You simply aren't going to get the same capabilities from a purely passive system than you will from an active + passive system where you can tightly control the frequencies and directions you are transmitting on, and have the information about what frequencies you transmitted on, when you transmitted them, and how (what array configuration) you transmitted them. The point is that the transmission itself is structured to reveal certain information. So when you get a response back (also, coming back to a phased system), you can learn far more about your target than you could in a purely passive system because you that prior information about what you transmitted.

I don't know where you are getting this 1km cluster idea from.

The point of cheaper radar systems is that you don't need to defend them as much, because destroying them requires equipment that's more expensive than the radar station. They overlap, and the locating that they provide can both protect the stations themselves and other targets in their coverage area.

How are those tanks going to maneuver undetected without air support?

I don't know, what's the point of the more expensive stations these are comparable to?