I believe EDI is the only remaining year round transatlantic destination that sees B752 service from EWR. I'd of thought they would want to get a Polaris/eco+ kitted out aircraft on the route sooner rather than later as demand returns, particularly with competition on the route from DL. The dispatch reliability of the B752 is far from great as well.
I'm surprised that they're putting the B764 on IAD rather than a widebody on EWR tbh, but I presume that all boils down to aircraft availability.