I think those machines aren't *real* broaching machines, but instead they have a mean to *simulate* the broach marks; how well that is done... is open to question... and there's the *age* issue; as a 40-50 yr old block shows other marks, crud, age, etc.. that they cannot replicate exactly (and even the accuracy of the broach marks they make is open to question). When I see a 40-50 yr old engine that the deck surface is clean sharp and/or *painted*.. those are all clues to look a little further..
My understanding is that GM's broaching machines weigh many tons... I doubt these 'replicators' have that sort of machine available to them, and if they did.. my question would be.. how much material do they 'broach' off to remove all traces of the original stamping, machining, etc..? I suspect they might fool *some people* with their approach... but not all..