September sales were actually up 13% from last year
Covid-19 shut down a lot of factories. Not just US car factories but also suppliers of small/cheap components (some of them worth less than one cent each) just couldn't be purchased in large volumes.
As far as I know the industry still hasn't fully recovered. 13% year over year growth isn't a 13% increase in demand, it's a 13% increase in the number of cars they're able to sell.