PAF is not going for anymore block52s, this news is either speculation or misinterpretation of arrival of four MLU F-16s from US, scheduled this Jan.
The F-16D 10801 is most probably on 'loan' to Lochkeed Martin, it has nothing to do US using Pakistan's paid F-16s...haven't you people seen AGM-65D and PAVEWAY-III undergoing flight trails on it? Its presence at FW is most probably a part of the Block52 and related equipment deal. Since these aircraft are tailor made for PAF, hence they have different versions of avionics as compare to block50/52s operated by other customers like Poland, Morroco, Greece, Oman or US. Weapon integration and flight trails on similar example block52 is a necessary condition before delivery to a customer.