The last time (I could be wrong) Pakistani volunteers (mostly Pashtun) in large numbers entered Afghanistan to assist the Taliban was in the 1996 period when the Talibs were much weaker and faced not only the Northern Alliance but severalUnreliable Indian sources estimate that more than 10,000 Pakistanis have entered the war zone in Afghanistan to openly support the Taliban's offensive against the Ashraf Ghani-led Afghanistan government. Pakistan's envisioned pivot to geo-economics continues to be a pipe dream due to their army’s addiction to sponsoring terror groups in Afghanistan and India.
anti-Taliban Pashtun groups in the Pashtun heartland but as you said the 10,000 figure didn’t come from a credible source. If anything I would guess that a lack of volunteers or manpower is not an issue for the Talibans at present but logistical and other issues.
As is well known Pakistan’s aims in Afghanistan are driven by the Indian threat (strategic depth) and also to ensure it has friendly relations with Afghanistan’s Pashtun majority; given that Pakistan has a large Pashtun population and unresolved border demarcation issues with Afghanistan along the Durand Line. India’s previous attempts to seek greater influence in Afghanistan and the reported support given to Baluch separatist groups have played to Pakistani paranoia.