Actually, Zhao I think you were right the first time. I just downloaded and checked the MIDI and it looks like to me the the Audio file is a rendering of the CSSS Solo Cello. I think the reason it's an audio file is because obviously noone has that library yet so it would be pointless supplying the MIDI alone (although the midi is there too).
And you're right, it sounds really really nice.
Here's a version of it with the solo cello boosted quite a bit to hear clearer...I added a touch of EMT 140 for a shiny reverb.
I read also it is a live cello in the recording