This is said with hindsight, it isn't information Stalin had access to at the time. His tendency to caution in foreign policy and his awareness of the American nuclear project certainly makes his decision to abide by the agreement pretty logical to my mind. Besides, a direct Soviet intervention in Greece may not directly lead to hostilities but would practically short circuit the agreements with the Western Allies before they had gotten started. With our knowledge of the Cold War, this doesn't seem like much but for the USSR this is still a period where a man like Henry A. Wallace may be president and the United States could accommodate the Soviets in the global order. Directly provoking the British like this and risking a war both sides considered very possible (Churchill's main fear vis a vis the EAM was an Anglo-Soviet War) would be both out of character for Stalin and incredibly risky with the information that was available at the time.