"Shipped and sold by Amazon" doesn't mean jack squat. They use their warehouses to commingle inventory from many individual suppliers.
People keep repeating this "not Amazon.com!" response but it hasn't been true for at least a year. Go look on /r/boardgames, /r/pcmasterrace, or that dude with the $5K camera that received a literal brick. These people post their invoices that clearly say sold by Amazon.com and people call them liars.
Of course I can't know for sure if it was that or comingling.
Amazon definitely puts 3rd-party returns back in stock if it looks like still sellable as new - the seller help site says "We evaluate the condition of each returned item. If we determine that the item is sellable, we will return it to your inventory". So they probably do that for their own items as well.