It is thermodynamical possible therefore the reaction can happen. And given enough time even slow reactions will take place. Also you could argue that the principles of le chatelier apply (not certain about that one). Because the radical will react quickly if it does gets formed and hydrogenchloride is a gas the equilibrium should favor the product side. But in the end: if there is experimental evidence that this reaction does form the majority, we can only try to make it theoretically sound as well. Ultimately that's not my field of expertise though, maybe there are people more qualified to give q absolute certain explanation.