You're confused by lots of things, Steven. So long as the Taliban don't start trying to export jihad to Chechnya or Xinjiang - which isn't really their style, despite what Western governments said to justify their invasion - then both Russia and China have an interest in engaging with the de facto power in Kabul. If the Taliban manage to hold on there, other governments will follow suit. Ordinary, grubby realpolitik...

As for Janos' original point, clearly the PDPA did retain a significant (albeit insufficient) base of support within Afghanistan and the military, which is why it was able to hold on for 3 years after the Soviet departure. The party was, after all, an authentically Afghan organisation, which had been strong enough to seize power in 1978 without external assistance.