I agree, Have to wait and see what the cap looks like. It will have to be mechanically fastened somehow I would think.
The cap is designed to slide onto the pipe and then clamp to the underside of the flange. It is fitted with a rubber seal that is supposed to pressure seal to the pipe when the cap is clamped down to the stack.
There will probably still be some leakage, but they are hoping for an 80%+ seal.