We know that there are Chinese muslims. There may even be a few of them who have crossed over the border to fight with the Taliban. What I don't believe is DEBKA's report that Beijing sent 5,000-15,000 of their muslim servicemen to fight alongside the Taliban.
It seems unlikely that any forces can escape to China now. First of all, I don't believe China wants these fanatics in their country, but more practically, it would be a trek of hundreds of miles through the highest mountains in Afghanistan to get there. We would be able to detect a troop movement of that size and I wouldn't like their odds.