Sinister has been cloning for years, long before HOX. We've seen them be failures like Madelyne (whom had no soul/consciousness of her own until the PF) and the Maraurders who we've seen as mindless husks in the cloning pens. My headcanon is that the meeting he had years ago as was seen in POX was about getting him on board and they used the Maraurders as a prototype for the Resurrections we see today. I believe that Xavier used Cerebro to try to capture and transfer the minds into those clones with varying degrees of success. The two worked together for years until Xavier had enough experience to take what he learned and applied it to the Five's procedure for a perfect resurrection