Nothing is too hot to handle for a big nation state like China. I don't know exactly why China didn't detain him. But there are a whole lot of possibilities from being to slow off the mark, avoiding ruffled feathers with the US, up to an agreement with Russia.

Maybe Snowden really hadn't anything with him in Hong Kong which he hasn't already shared with Journalists and the Russians.

I've assumed that both China and Russia have access to his complete unfiltered information in exchange for his safety.

Hard for Snowden to say no to those demands at that point. "Give us everything you have right now and we won't send you back to the US."

China does the same for any equipment or planes they seize from the US- inspect the hell out of it and then return it.

"avoiding ruffled feathers with the US" is what I meant by China deciding his data was too hot to handle. I agree with the other possibilities, too.