In the moutains 150K really isn't that much. You would need alot just of service and support troops for an endevour like that.
Plus, Winter is coming along now. Why would you want to start something in the winter ?
The only Army that would want or need to fight in the winter is NK. Nov and April are the key times for them to move South. Maybe the CHICOMS are just saying no with a little bit a backing.
Large buildup or small, North Korea doesn't have the resources to defend their western border and the DMZ. The troops in place on the west are mostly to stop people from leaving. Were the Chinese so inclined, they could push through the border and make it deep into North Korea before running into any significant resistance. If they are able to convince a few key commanders not to oppose them, they could make it close to Pyongyang in short order.
There are a number of reasons why this might be desirable for both sides. Most likely would be the imminent collapse of the government. The leadership of the DPRK could flee to China, and the PRC wouldn't have to risk Korea being unified by the ROK/USFK military.