I get the sense that any formal request for help from China, made public, is more of a warning than anything. China has been sick of North Korea's antics for a while now, so any public talk between the US and China is both countries' way of saying, "You're honestly ticking me off now." It's more powerful coming from China, considering China's general support of North Korea. I'm sure China has no real interest in working with the US when it doesn't have to, but it suits its needs to talk like it may.
The US benefits by making a show of asking, China benefits by letting North Korea consider it potentially helping, and everyone goes home happy.