How does one respond to the specious argument that the four Imams did not know the sound (sahih) hadith from the weak (da'if), and that they deemed hadith weak or sound using a method different from those who abolished the schools of jurisprudence for the sake of a claimed Salafism?

The four Imams and other Imams of Islam knew the authentic (sahih) hadith from the weak (da'if), differing in this due to their varying opinions on some narrators, the existence of corroborating evidence for a hadith, or the acceptance or rejection of mursal (missing first part of chain) narrations. The verifiers (muhaqqiqoon) who came after them followed their methodology, weighing and preferring among their statements. So, whoever follows their methodology is not to be blamed. However, whoever deviates from it and adopts another methodology that contradicts theirs and their principles is a falsifier (mubtil). Judging an individual or a group as having deviated from the methodology of the Salaf requires study and verification.