Honestly, I think sending troops to Korea was going to be an attractive option for Mao no matter what the US and UN forces do or don't do. Remember, the Chinese Civil War only ended the year before. Millions of soldiers needed to be demobilized. Any time a state has many men who have been trained to kill, and may not be able or be willing to rejoin civillian society, it is a very dangerous time for that state. It can easily result in brigand armies, or even rebel forces.
It's the same reason (but of course not the only reason) that Toyotomi sent his armies to Korea, it's part of the reason that Spain sent soldiers to the New World, or why Weimar turned a blind eye to Freikorps in Eastern Europe. Don't get me wrong, if the US/UN looks like it will encroach upon the Yalu, or if the US offers the KMT shelter outside of China in Taiwan, it will make the PRC will be more likely to act. But finding a use for those troops, including it must be said possibly seeming some of them die off, has a logic all its own.