Each run of a random quantum circuit on a quantum computer produces a bitstring, for example 0000101. Owing to quantum interference, some bitstrings are much more likely to occur than others when we repeat the experiment many times. However, finding the most likely bitstrings for a random quantum circuit on a classical computer becomes exponentially more difficult as the number of qubits (width) and number of gate cycles (depth) grow.
It sounds like they're saying "simulating this physical process accurately is extremely computationally expensive, but just running the physical process in the real world is MUCH faster". But that's true of basically any physical process!
Simulating turning on a single lamp in a room and figuring out how much light falls on what walls is an enormously computationally expensive process, which can take immense amount of time to figure out (depending on your desired level of accuracy). Movie studios, CG artists and game companies build entire render farms to solve this problem. But the physical process itself (just turning on the light) happens instantly. Just replace "turning on the light" with "running a quantum circuit".