I noticed the "only one on sale" thing too. I suspect the author meant it was the only one for sale at the store, period, not that it was the only one with a sale/discounted price.
Now I'm just looking forward to see how more and more ubiquitous AI deals with real world language.
When I look up "on sale" I get
> 1. offered for purchase.
> 2. NORTH AMERICAN - offered for purchase at a reduced price.
so I guess I associated a US centric idea with the term, since that is what I'm used to.