Stalin knew because Dean Acheson had said so publicly. Stalin probably also knew the state of American unpreparedness in 1950.
Truman had taken a meat ax to the defense budget and our military was in terrible condition. MacArthur had one full strength infantry regiment under his command - the segregated, all black, 25th Infantry. Unfortunately the 25th had the worse equipment in the army and was used as a dumping ground for incompetent white officers because the army thought blacks would not fight. Racism, both disparagement of blacks and underestimation of the Chinese cost us thousands of lives in that war.
The rest of MacArthur's command was under strength and out of shape from four years of garrison duty in Japan One example is the first US tanks to engage the North Koreans, M24 Chaffees. They fired one round and were disabled because they had no hydraulic oil for their recoil systems and their guns came off the trunnions and sort of rattled around inside the turrets. Seems Truman's pal defense secretary Luis Johnson had seen fit to cut recoil oil out of the budget. After the shooting started and money was no longer a problem MacArthur rounded up some old M4E8s that were rusting away on Guam and sent them to Japan where Mitsubishi was able to refurb them and send them into action.
The Truman administration had let our defensed down and Stalin made us pay. Stalin was probably justified in assuming the US wouldn't fight with an army in the shape it was in. He just underestimated the arrogance of MacArthur, the incompetence of Truman and the speed with which the US military was able to and mobilize and react.
It was the 24th Infantry Regiment of the 25th Infantry Division that you are referring to.
The 25th Infantry Division was composed of the 19th Infantry Regiment. the 24th Infantry Regiment and the 34th Infantry Regiment.