I've never read anything to that effect, for what that is worth.

During the Korean War, there was an impression given of through some ambiguous press conferences that the United States was willing to use nuclear weapons against China should it escalate the war, but these were misconceptions/bluffs. However, during the Taiwan Strait Crises of 1955-1958, there were overt threats, in the sense that top US military policymakers talked quite openly about using nuclear weapons. All of these acted to accelerate Chinese interest in their own nuclear arsenal.

But I have not heard of any threats made in 1949. It would be somewhat uncharacteristic given the general ambivalence of Truman towards nuclear weapons use, and because it would have taken considerable effort to deploy said weapons over there (this would have been before US nuclear weapons were globally deployed). There was talk, internally within the US, of possibly using military force to repel a Chinese invasion of Taiwan in 1950 — but not with atomic bombs.

If you are asking because the question of "why didn't China take Taiwan then?" is what you are really after, the conventional answer is that this had to do with China failing to take Kinmen, which put its invasion plans on hold while it reassessed the situation. The beginning of the Korean War then redirected their attention elsewhere.

There are many sources on US nuclear threats towards China in this period. Nina Tannenwald's The Nuclear Taboo looks at the Korean War and Taiwan Strait Crises, for example. Bruce A. Elleman, Taiwan Straits Standoff, chapter 6 ("The US Threat to use Atomic Weapons") describes the later threats in details.