If it was planted there to incriminate Oswald that would prove that the conspirators had already designated him as the patsy. That wouldn't fit the argument here about that being decided after the assassination.
That is Alan's argument, no-one else's.
It's apparent from Oswald's reported behaviour before and after the assassination that he was involved in something for which he expected to be arrested (leaving a relatively large amount of cash behind as he had reportedly done before the Walker attempted assassination, leaving his weeding ring behind fits into this narrative as well).
If not the actual assassin, he was heavily involved with the events of that day. If the intention was to set Oswald up as the patsy all along, it is not a stretch to imagine he was under orders to stay out of sight around the time of the assassination.
The idea that it was decided, after the event, to set up Oswald without any pre-planning is seriously undermined by the Backyard Photos (which have been authenticated). They show Oswald holding a rifle indistinguishable from the rifle Tom Alyea filmed being removed from it's hiding place on the 6th floor.
All that about the inside of the barrel being rusty.
I'm pretty sure the upshot of that discussion was that the barrel was not rusty but did show signs of corrosion (which isn't really the same thing).