I don't believe that is true . Powders come in very different burn rates . This means you can get the same velocities with quite a bit different barrel times resulting in POI shifts . Its the very reason two different factory loads can have the same velocities with different POI .
As to the OP , what do you mean by defensive loads . Yes I understand the general point but specifics can help narrow down the answer . Meaning just about all defensive incidents happen well inside 200yds and likely inside 100yds so the question I have is . How big is your POI shift you are trying to reduce for lets say inside of 100yds ? Although I have had some big POI shifts from load to load . Those often are because the bullets are very different . When using the same bullets my POI shifts at 100yds tend to be at most 1" with the same premium bullets . That's plenty close enough in a real world someone shooting at me need to return fire as fast and accurately as I can situation . At the range shooting steel at 100yds , my sub moa loads just don't ever seem to shoot sub moa with rapid fire off hand , I'm lucky if I get 6moa
So what I would do is try to find the same bullet and powder the manufacture uses . Then do a load work up that has the closest POI . I'd say if you can find anything that both shoot inside the same 2" circle at 100yds regardless of where inside that circle , you are GTG in a real world situation .