My guess he is either talking out of his ass, or getting confused regarding the ATF's determination that the rifle needed to be complete with requisite "evil" features before sept 94 in order to be grandfathered.
Likely, these were never complete rifles and so, according to the ATF, you can't assemble them in a preban configuration. The "replacement lower" thing seems hogwash - as mentioned before, if your preban lower is damaged beyond repair, it needs to be replaced by the original manufacturer. Since it really doesn't cost them any more to do this than replace a postban receiver (except maybe some more paperwork and special attention re: restamping the original ser #), you likely wouldn't need to pay the premium for a preban receiver that this guy is obviously charging.
Who knows if he is being purposely deceptful or just stupid, however. I would pass on the deal, in either case. Keep in mind though, that the whole "must be a complete rifle" thing is just the ATF's opinion of the law - it has never been tested in court, where it really matters, yet. However, the price he is selling these for probably isn't a good enough deal to risk that you'll be the first to test it in court. OTOH, wasn't it Oly who had the fire and all its records destroyed? So, no one could really prove one way or the other whether this left the factory assembled or stripped.
Rocko