To weaken and to test.
Further, China has their own muslim maniacs to contend with and this may be part of a "diplomatic" effort to coax the Afganistanis out of committing societal suicide. (too late however, the deed is done).
I don't believe for a second that when push comes to shove that the chinese are going to waste one box of bullets defending the taliban and risking war with their largest economic trading partner. They have just learned "materialism" and they like it, they love it. They want and need peace for several more years at least. So relax. China is not going to help them.
uh-oh...