"Potential entities" here doesn't mean "currently existing non-morally-significant entities that might give rise to morally significant entities", just "entities that don't exist yet". A much clearer phrasing would be something like "Does my utility function aggregate over all entities existing in spacetime, or only those existing now?" IMO, the latter is obviously wrong, either being dynamically inconsistent if "now" is defined indexically, or, if "now" is some specific time, implying that we should bind ourselves not to care about people born after that time even once they do exist.