It’s a chainstay that’s not long
Depends on who you ask, but I’d say anything longer than 435mm is not short.
My preference is 420-425, wheel size, motor placement, and suspension dictates length, though some builders are lazy and just make the CS long cuz it’s easier and the average rider won’t know better.
A shorter chainstay makes a bike more maneuverable, easier to manual, shorter wheelbase leads to a quicker bike. No real downsides thoug some folks will argue differently.
I’ve ridden 29ers with a 415mm chainstay and it’s was quite rideable at all speeds in all conditions. Makes a bike much more fun and playful. More poppy? Yes.