Based on their claims of how they validated it by comparing the data with 60 some persons whos tax filings were public (e.g. politicans) it would be reasonable

Based on their claims of how they validated it by comparing the data with 60 some persons whos tax filings were public (e.g. politicans) it would be reasonable to assume that ProPublica received detailed tax records on a considerable fraction of all Americans. Certainly whomever collected the data had access to that.