The key claim is linear scaling. IBM: "The runtime of our simulation method scales approximately linearly with the circuit depth". That directly contradicts what Google wrote in the paper: "... algorithm becomes exponentially more computationally expensive with increasing circuit depth".
I'm sure you mean infeasible. "Impossible" goes back to a problem's decidability. If a problem is undecidable, there's no way you're going to find its solution by switching computational models; classical computers can do anything a quantum computer can do, just "slower".
Not if they use up all the energy in the visible universe or they need so much power they collapse and form a black hole.
Real quantum computers should be able to do calculations that are that powerful, assuming we can prove the best classical algorithms are really exponential.
The quantum circuit based simulation actually has much worse accuracy. A classical computer can simulate the class of circuits they are proposing with linear increasing runtime (as opposed to exponential as Google claims) and _much_ higher accuracy according to IBM.